Difficulty

Easy

 * The Tank has 3000 health points.
 * The Witch has 500 health points.
 * Friendly fire does not inflict damage, but friendly fire incidents will still register and disqualify players from earning the Safety First achievement.
 * Common infected inflict 1 point of damage per hit, 2 if attacking a survivor's back.

Normal

 * The Tank has 4000 health points.
 * The Witch has 1000 health points.
 * Friendly fire inflicts 1 point of damage per hit to the survivors.
 * Common Infected inflict 2 damage per hit from the front, and 4 from the back.

Advanced

 * The Tank has 6000 health points.
 * The Witch has 1000 health points.
 * Non-headshots get no bonus damage
 * Common Infected inflict 5 damage per hit from the front, and 10 from the back.
 * Friendly fire inflicts 50% damage per hit to the Survivors.

Expert
This is a mostly complete list, extracted in part from the exposed cvar interface. See Console commands for information on the cvars.
 * The main attack of the Witch is a one-hit kill.
 * The Tank has 8000 health points.
 * The Witch has 1000 health points.
 * The Tank takes longer to burn when set on fire.
 * Fire inflicts more damage to the Survivors.
 * Friendly fire inflicts full damage to the Survivors.
 * Common Infected inflict 10 damage per hit from the front, and 20 from the back.
 * First-aid kits are replaced by pills.
 * First-aid kits are never found outside of Safe rooms and finales.
 * Minimum interval between Horde attacks is much shorter.