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. 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.

If players play using this method, the use of dual-pistols and scoped rifles is recommended as Common Infected are quite weak, and a shot kills them instantly without burst firing on them.

Details

 * The Tank has 3000 health points, does 24 damage per hit and 75 when pounding.
 * The Witch has 1000 health points and is startled in 8 seconds. She does 12 damage on an incapped Survivor every half of second (every 5 seconds in L4D2).
 * 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. The damage will also be count into Steam player's statistics on PC.
 * Molotovs, Gas cans, Fireworks, and Explosive Barrels, once ignited, can still cause damage to you and other Survivors, so watch where you throw or shoot them. Note that a Survivor Bot can accidentally hit Gas cans, Fireworks, Explosive Barrels and deal damage to you and the team. The worst thing that can happen is the fire incapacitates you, then the Survivor Bots run into the fire to save you, causing them to get incapacitated if low on health.
 * Pipe Bomb, Oxygen tank, Propane tank, and Grenade Launcher will not do damage to your teammates, but will do roughly 5 to 30 damage to yourself if detonated too close. A Survivor Bot can also accidentally shoot Oxygen tank or Propane tank and damage him/herself.
 * In The Sewer, Survivors can incapacitate or kill themselves by shooting the gas station.
 * Common Infected have 30 health and deal 1 damage per hit, 0.5 if attacking a Survivor's back.
 * There is roughly a 1/5-second grace period where the Survivor will not take damage after being inflicted upon. This only affects damage dealt by the Common Infected.
 * The Smoker deals 10 damage per hit on ensnared Survivors and has a 5 seconds reaction time before attempting to ensnare a Survivor.
 * The Hunter deals 10 damage per claw and 5 damage every half-second to pounced Survivors.
 * The Charger deals 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 32, the body acid's average damage is 32 and does 1 damage per hit.
 * The Boomer will not vomit as soon as it sees you, possibly around a 5-7 seccond reaction time before vomiting.
 * Special Infected such as the Smoker take longer to aim before attacking, and Special Infected usually don't sneak up on you, rather then specific Smokers.

Normal
Normal mode is fairly balanced, as far as the difficulties go. It is 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 and Survival mode. Differences between other modes and Versus will be stated in parentheses.

Details

 * The Tank has 4000 (6000 in Versus) health points, deals 24 damage per hit and 75 (24 for Versus) damage when pounding.
 * The Witch has 1000 health points and is startled in 5 seconds.
 * Friendly fire inflicts 10% of the weapon's damage to the Survivors.
 * Common Infected have 50 health and inflict 2 damage per hit in the front, 1 from the back.
 * There is roughly a 1/4-second grace period where the Survivor will not take damage after being inflicted upon. This only affects damage dealt by the Common Infected.
 * The Smoker does 10 damage per hit on ensnared Survivors, 2 damage per hit (and 3 while dragging in Versus).
 * The Hunter deals 10 damage per claw and 5 damage every half-second to pounced Survivors.It also does a certain amount of damage on a pounce in versus depending how far away and how high you pounce from.
 * 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, the body acid's average damage is 69 and does 2 damage per hit.

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 exacerbate the problem further, unless the Director feels generous. Tanks can cause huge amounts of problems as their health is twice as much, causing Survivors to adopt different tactics for dealing with it.

Players are highly recommended to use melee weapons with an AK-47 or scoped rifles, as even with a shot with a Magnum sometimes doesn't manage to kill the Common Infected quite easily. Melee weapons can easily decimate a horde faster then pistols, which saves health in the long run. Depending on luck, the number of Tanks and Witches, players should expect to be finished within one hour.

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. She also does 68 damage to an incapped player every half of second (every 5 seconds in L4D2), making it dangerous if you have no assistance and she has full health.
 * Molotovs and other explosives deal more damage onto allies then before. Careful where you throw them!
 * Non-headshots get no bonus damage.
 * Common Infected have 50 health and inflict 5 damage per hit from the front, 2.5 from the back.
 * There is roughly a 1/4 second grace period where the Survivor will not take damage after being inflicted upon. This only affects damage dealt by the Common Infected.
 * Friendly fire inflicts 50% of the weapon's damage to the Survivors. Be careful with firing!
 * The Smoker does 20 damage per hit on ensnared Survivors and 5 damage per claw.
 * The Hunter deals 20 damage per claw and 10 every half-second on pounced Survivors.
 * The Charger does 15 damage in a punch, 15 damage when its target reaches a wall, 15 damage per slam and 2 damage if Charger missed his target, but stumbled him.
 * The Jockey does 8 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 damage per hit.

Expert
Expert mode is 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 be killed here. But then even moving has its downsides. 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, excluding the Witch who can instantly kill a Survivor in one hit, but can be avoided.

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, though it is quite rare for the final Tank to not incapacitate anyone and having all Survivors get into the escape vehicle. There are also many unique nuisances in Expert difficulty that you won't see in the others.

Every Special Infected must be taken seriously due to their faster reaction times upon approaching the Survivors. Chargers must be taken care of before they start sending Survivors with them into the water, and its best not to split up, as Special Infected can get rid of a Survivor in ease with their extremely high amount of damage.

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 attempting to deal with the Infected.

Gauntlet Crescendo Events such as the Tower Event on The Parish can cause players great difficulty, due to the large amounts of hordes that come even on Easy difficulty.

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.
 * The Tank has 8000 health points, incapacitates Survivors in one hit and deals 150 damage when pounding (killing an incapactitated Survivor in two hits).
 * Fire inflicts 40 damage per second to the Survivors. Be careful with Molotov throwing.
 * 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 have 90 health and deal 20 damage per hit from the front, 10 from the back.
 * There is a one-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 both the Common Infected and the Special Infected, yet it is almost unnoticeable in case of Boomers (as they can be killed quickly shooting at any part of their body due to their very low health) and it does not apply at all to Tanks.
 * 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. However, health packs may spawn in the health closets if the team has been seriously hurt and possess limited or no forms of healing.
 * The Smoker does 30 damage per hit on ensnared Survivors, does 20 damage per claw and takes 8 seconds to kill a full-health Survivor.
 * The Hunter does 40 damage per claw, 15 damage every half-second to pounced Survivors and takes 9 seconds to kill a full-health Survivor, or incapacitate them in 3 seconds if clawing.
 * The Charger does 20 damage in a punch, 20 damage when its target reaches a wall, and 15 damage per slam.
 * The Jockey does 12 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 Infected to spawn on the paths which you have already taken.

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 an 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 been 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 with a Defibrillator.
 * Witches will kill any Survivor they hit, regardless of the difficulty (not on easy); hence, The Sugar Mill requires extreme caution on Realism. If a Witch incapitates you on Easy, after revival, a small chunk of your health is taken away.
 * Witches cannot be cr0wned with any type of shotgun, forcing the use of different tactics such as ammo upgrades, burning and running away, and more. See the Witch's page on killing her.

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. It 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 use vocalization commands (Z and X by default on PC) or use microphones to inform each other if they are in trouble.

If playing with Bots, nothing will happen. This is because the Bots will follow you at all times, so if one player is struck by a Special Infected, they will come to your rescue. This way, players won't need to talk with each other as much when being attacked by Special Infected, and their accuracy aganist Smokers and other Specials is superb to that of a player.