Difficulty

There are four difficulty levels in Left 4 Dead. Each level refers to the difficulty of the campaign, and each has drastic differences over the next. For example, on the Easy difficulty, it's rare to see a horde outside of the occasional Crescendo Event, whereas on Expert it's likely you'll face a horde around every corner. The specific differences between each difficulty level are listed below.

Easy
As the name suggests, this difficulty is the easiest of the four. Players receive an abundance of health packs. Common Infected die from a single pistol shot, meaning that a single player with a pistol can decimate a horde in seconds. Shotgun shells are able to kill Infected at sniper range. This mode would be recommended for beginners and those who simply want to relax. This is also an easier way to get achievements without the trouble of the harder difficulties.

Details

 * The Tank has 3000 health points, does 24 damage per hit and 75 when pounding.
 * The Witch has 500 health points and is startled in 8 seconds.
 * Friendly fire does not inflict damage, but friendly fire incidents will still register and disqualify players from earning the Safety First achievement in Left 4 Dead.
 * Molotovs and Gas cans, once ignited, can still cause damage to other Survivors, so watch where you throw them. In addition, detonating a Propane tank, Pipe bomb, or Oxygen tank will also do friendly fire damage. Also note that a Survivor Bot can accidentally hit one of these items and deal damage to you, incapacitate or even kill you, depending on your health.
 * Common Infected inflict 1 point of damage per hit, 0.5 if attacking a Survivor's back.
 * The Smoker does 10 damage per hit on ensnared Survivors and takes 5 seconds to ensnare a Survivor.
 * The Hunter does 10 damage per hit and 5 per hit on pounced Survivors.
 * The Charger does 5 damage in a punch, 10 damage when its target reaches a wall, and 15 damage per slam.
 * The Jockey does 3 damage per hit while riding a Survivor and does 1 damage per hit.
 * The Spitter's spit acid's average damage is 31 and the body acid's average damage is 32 and does 1 damage per hit.
 * Average chances of survival: 4 survivors=100% 3= 75% 2=50% 1=25%

Normal
Normal mode is fairly balanced, as far as the difficulties go. It's harder than easy, but not something that will quickly decimate an inexperienced player, such as in advanced or expert.

Normal is also the set difficulty for Versus mode. Differences between other modes and Versus will be stated in square brackets.

Details

 * The Tank has 4000 (6000 for versus) health points, does 24 damage per hit and 75 [30] damage when pounding.
 * The Witch has 1000 health points and is startled in 5 seconds.
 * Friendly fire inflicts 10% of the weapon's damage per hit to the Survivors.
 * Common Infected inflict 2 damage per hit in the front, and 1 from the back.
 * The Smoker does 10 (3 while dragging and 10 when survivor is constricted) damage per hit on ensnared Survivors and 2 damage per hit.
 * The Hunter does 10 damage per hit and 5 per hit on pounced Survivors.
 * The Charger does 10 damage in a punch, 10 damage when its target reaches a wall, and 15 damage per slam.
 * The Jockey does 4 damage per hit while riding a Survivor and 2 damage per claw.
 * The Spitter's spit acid's average damage is 68, and the body acid's average damage is 69 and does 2 damage per hit.
 * Average chances of survival: 4=80% 3=60% 2=40% 1=20%

Advanced
Advanced is for more experienced players. Although it's not as devastating as Expert, it demands a high level of competency and care. If the player is not careful, then they should be ready for multiple tries per map, as things can get pretty hard — and friendly fire incidents are the bane of a serious player, as they take off quite a bit of health here. The fact that first aid kits don't spawn very often only serve to further exacerbate the problem.

Details

 * The Tank has 8000 health points, does 33 damage per hit and 100 when pounding.
 * The Witch has 1000 health points and is startled in 5 seconds.
 * Non-headshots get no bonus damage
 * Common Infected inflict 5 damage per hit from the front, and 2.5 from the back.
 * Friendly fire inflicts 50% of the weapon's damage per hit to the Survivors.
 * The Smoker does 20 damage per hit on ensnared Survivors and 5 damage per claw.
 * The Hunter does 20 damage per hit and 10 per hit on pounced Survivors.
 * The Charger does 15 damage in a punch, 15 damage when its target reaches a wall, and 15 damage per slam.
 * The Jockey does 6 damage per hit while riding a Survivor and 5 damage per claw.
 * The Spitter's spit acid's average damage is 68, and the body acid's average damage is 69 and does 5 damge per hit.
 * Average chances of survival: 4=60% 3=45% 2=30% 1=15%

Expert
Expert mode is, obviously, the toughest difficulty of the four. It is not at all recommended for the inexperienced or impatient, as it may take many tries for the player to complete a single chapter. In this mode, never fear death, and always be on the move. Standing still is an almost guaranteed way to get killed here. But then, even moving has its down sides. The player must be ready for anything, anticipate everything. However, they must remember that in the end, it is very likely that they will die. Though it can get incredibly frustrating at times, with practice and a great deal of patience, it is possible to get through. The Tank is the worst thing that could happen on Expert.

Normally finishing an entire campaign in Expert difficulty may require 2 to 6 hours (in extreme circumstances) of gameplay, and only one or two Survivors can make it to the escape vehicle. There are also many unique nuisances in Expert difficulty that you won't see in the others.

The horde
The time between Panic Events in Expert mode is a lot shorter than in the other modes, with around four hordes in three minutes, and the size of the horde is bigger. On top of that, Boomers will constantly be sent at the Survivors (though not as much in Left 4 Dead 2 due to more Special Infected), vomiting and exploding all over them. This makes progressing somewhat difficult as they may be stuck within one area for long periods of time attempting to deal with the Infected.

Details
This is a mostly complete list, extracted in part from the exposed cvar interface. See console commands for information on the cvars.
 * The Witch has 1000 health points, can instantly kill a Survivor and is startled in 4 seconds. Witches can only be CR0WND with a headshot (on easier difficulties you can shoot her in other parts of her body and still 'CR0WN' her) from a Pump Shotgun or an Auto Shotgun. If Witches are avoided without being startled the Director ensures to continually spawn them in your path, sometimes spawning three or four per level if the player(s) manage to keep avoiding them.
 * The Tank has 8000 health points, instantly incapacitates Survivors in one hit and 150 when pounding.
 * Fire inflicts 40 damage per second to the Survivors.
 * Friendly fire inflicts full damage to the Survivors. Avoid friendly fire at all costs. One direct shot from a Pump Shotgun or Auto Shotgun will instantly incapacitate a teammate.
 * Common Infected inflict 20 damage per hit from the front, and 10 from the back. Unlike the other difficulty levels, there is a 1 second grace period where the survivor will not take damage after being inflicted upon. This only affects damage dealt by the Common Infected.
 * Non-headshots deal far less damage to the Common Infected. However, whether this case applies to the Special Infected requires confirmation.
 * First aid kits are extremely rare on Expert outside of the safe room and before the finale (except for the Death Toll Finale, and in the Ambulance right outside of Mercy Hospital.); They are instead replaced by Pain pills, which become surprisingly common.
 * The Smoker does 30 damage per hit on ensnared Survivors and does 20 damge per claw.
 * The Hunter does 40 damage per hit and 15 damage on pounced Survivors and takes 9 seconds to kill a Survivor even if they has full health.
 * The Charger does 20 damage in a punch, 20 damage when its target reaches a wall, and 15 damage per slam.
 * The Jockey does 9 damage per hit while riding a Survivor and 20 damage per claw.
 * The Spitter's spit acid's average damage is 68, and the body acid's average damage is 69 and it does 20 damage per hit.
 * At close range the Tank is extremely good at cutting off and closing in on Survivors who try and turn, even a little (by turning it any movement other than running directly away from him). The only way to outrun a Tank on Expert at close range is to continue to run directly away from him. The same goes for his rock-throwing attack, it is absolutely unavoidable if you are not paying attention. The best way to avoid his rock throw attack is to continue to move in one direction while he prepares to throw the rock, then when he throws it change directions quickly. Or simply remain still and move at the right time. Another good strategy is to simply place a solid object like a wall or car between yourself and the incoming rock.
 * The Boomer appears to have noticeably increased range with his vomit attack and it does 20 damage per claw.
 * Enemy reactions are much faster; for example, Smokers will often pause for several seconds on easier difficulties before snaring you with his tongue; however, on Expert, they won't hesitate to attack again.
 * The most threatening Special Infected will almost always spawn on the path(s) you must take, namely the Tank and the Witch. However, observant players might notice a roaring, muscled or hunched, crying mass in the distance, and closed captions can help predict enemy locations, which can help a player get the drop on the Special Infected.
 * Players are penalized more severely if they wander off course, back-throttle, or stagnate at a location, causing zombies to spawn on the paths which you have already taken.
 * Average chances of Survival=4=40% 3=30% 2=20% 1= 10%

Important Tips

 * Play with skilled and cooperative teammates and use microphones.
 * Establishing a good rapport with your teammates helps to foster a good relationship and enforce loyalty, reducing the chances of players quitting abruptly (rage-quitting) and disrupting gameplay.
 * It is advantageous to test run with a few friends on a map with a slightly lower level of difficulty so as to gain the opportunity to learn when and what to anticipate from your teammates on the intuitive level, as well as gauge their pace, in preparation for Realism settings.
 * When playing on higher levels of difficulty, especially on Expert or Realism settings, it is advised to have good ping (green or yellow bar) while making sure that whoever hosts the game has a good system, as not having a good system can result in lag. Even the slightest delay as caused by laggy framerates or a stuttering connection will mean the difference between being instantly killed by a Witch, or managing to evade her within a split second.
 * It is better to play with three human players than Survivor Bots, not only can the game not continue if all human players are dead even if some Bots are still alive, but also the human players are more cooperative and can pick up Defibrillator units (which are important in this difficulty). Survivor bots also do not know how to pick up and utilize tertiary weapons such as pipe bombs. However, one interesting advantage is that Survivor bots do not seem to deal friendly fire damage even if they hit you.
 * It is suggested that the first player play as Zoey or Ellis, as they use sniping rifles when controlled as Survivor Bots. The Sniper Rifle and Hunting Rifle don't work very well in this difficulty, since Special Infected often attack immediately on sight, and Survivors won't have time to stop and snipe.
 * One Survivor making it into the safe room is enough to advance to the next chapter. Any Survivors who didn't get in the safe room will respawn in the beginning of the next chapter with 50 permanent health, which is usually higher than the health before they get into the saferoom, the only downside for this is all items will be lost.
 * However, this can be used to the Survivor's benefit. Simply kill the Survivors that make it into the safe room with less than 50 health, and then close the door to advance to the next chapter. All items will still be spawned inside the safe room, and the dead Survivors will respawn with 50 permanent health (and carry over health kits and other items they had before being killed). However, if not given permission by the other players (if any), you can get voted out of a game for team-killing.
 * Use items for temporary health as much as you can, save your health kits on those Survivors whose would die from being hit once.
 * Stay away from open areas, use your Pipe bombs and Bile bombs if you have to fight hordes in open areas. The open areas are only good when you are fighting Special infected such as Tanks.
 * Think before setting the Tanks on fire in this difficulty on Left 4 Dead, as it increases the Tank's speed but burns its health away via a timer. If the Tank is on fire, consider evading him rather than engaging him, but watch your movement as getting stuck on obstacles can get you killed very quickly.
 * In Left 4 Dead 2, Tanks are slowed down like in Versus on Left 4 Dead, and their health being burned away before death is based on their current health.
 * Pick up Defibrillators and Ammo upgrade packs after you used your kit. Incendiary ammunition are good for Tanks while Explosive ammunition are good for Witches and other Special Infected. Deploy them with your teammates before you fight with them. However, be careful with the explosive rounds, as they will damage you and your teammates a lot.
 * Attaching Explosive ammunition to a rapid-fire weapon such as a Submachine Gun or Assault Rifle is a good idea as it often takes just one or two bullets to stumble a Special Infected. This is extremely useful especially against the Charger who cannot be shoved off a Survivor as one Explosive bullet can stumble the Charger before he can start pummelling his captive. However, be warned that this causes friendly fire, and this can be deleterious on Expert settings.
 * Be careful when the rescue vehicle arrives, Survivors often rush to the vehicle rather than risking their lives to save their teammates.
 * Think before picking up a super weapon such as a grenade launcher or a chain-saw. In spite of their vast firepower, the need to frequently hold back due to the avoidance of friendly fire inhibits their utility and forces you to stick to melee or your secondary weapon much of the time.
 * Do not expect lines to be drawn between who should be responsible for looking out for each other, or who should be on the offensive. Expect to take up all different roles of gameplay in almost all situations.
 * While it is possible for a gung-ho player to rush ahead of his or her teammates to complete the campaign as soon as possible, this can sometimes be ill-advised on higher levels of difficulty, where temporarily separation entails permanent segregation. It also yields a bad impression to certain players.

Realism
Realism is meant for people who play frequently together online. It can be activated on any difficulty level and changes gameplay to become, like the name suggests, more realistic, or in the developers' words, "like in the movies". Realism can't be accessed in Left 4 Dead as it is a exclusive mode for Left 4 Dead 2. Playing on Realism at higher levels of difficulty like Expert is ill-advised until you have garnered sufficient practice to grasp an intuitive feel and instinctive anticipation of other players' behaviors. Thus, it is recommended that players should familiarize with each other before attempting Realism.

Details

 * Only headshots are instant kills, and attacks on the torso or limbs cause much less damage than they would normally do. Direct hits with the Grenade Launcher, Magnum, and M60 are still one hit kills.
 * Realism removes the outlines that normally appear around teammates when there is an obstruction between one another. Similarly, outlines around weapons and items also do not appear unless close enough to grab them. (Items that would normally have blue outlines have no outlines, making it harder to notice a lesser Tier weapon as well.) Outlines around Special Infected, such as a Hunter that pounced a Survivor, are also removed. This makes it particularly difficult for players to keep track of each other, and friendly fire will be much more difficult to avoid.
 * Only white and orange outlines remain.
 * White outlines are around items that are within reach, as well as Gas cans that have not being picked up in The Atrium] and The Port.
 * Orange outlines are around Gas cans that have being picked up and dropped during the The Atrium] and The Port
 * You do not respawn in closets so if you die, you must either wait until the next chapter or be revived via a Defibrillator.
 * Witches will kill any Survivor they hit, regardless of the difficulty; hence, The Sugar Mill requires extreme caution on Realism.

Some possible strategies to cope with the increased difficulty is to have at least one player carrying a Defibrillator along, and consider killing all members below 50 health in the safe room so that they will respawn with 50 health in the next map, although keep in mind that sidearms may be lost (i.e. Magnums are relatively rare and are only found a few times in one campaign) or possible misunderstanding of team-killing by other players. Also is recommended to do this only in safe-room, as the "main" Survivor can be incapacitated after team-killing. Players might also find it useful to bind their keys to vocalize commands or use the microphone to inform each other if they are in trouble.