Certain attacks and conditions can either drain the character's ability scores or effective level. These types of attacks are typically the result of hostile spells such as Ray of Enfeeblement or Enervation. They may also be the result of attacks from certain Undead creatures such as Vampires or Wraiths.
When a character is successfully affected by an ability drain, the attack will supernaturally lower one or more of the character's ability scores (such as Strength or Constitution). The effect typically does not end upon resting unless it is a temporary effect (such as from the spell Ray of Enfeeblement).
When a character is successfully affected by a level drain, the attack will supernaturally lower the effective level of the character. While his actual level remains intact, he combat ability (AB, saving throws, effective caster level, etc.) are lowered by the amount equal to the number of levels drained. The effect typically does not end upon resting unless it is a temporary effect (such as from the spell Enervation). More negative levels than character levels causes instant death.
Removal of ability score affecting damage is typically accomplished by casting a Lesser Restoration spell (or better) upon the victim. Removal of certain effects may require the use of the Remove Curse spell instead. Level draining can only be removed by a Restoration spell or better upon the victim.