Negative levels (level drain): Certain monsters are able to sap the life essence of other creatures. The draining effect of each negative level applies a -1 modifier to attack rolls, skill checks, caster level, DC of spells, and saving throws. If a creature's negative levels ever meet or exceed their current level, they die.
Though the character sheet shows a decrement to class levels, this only impacts regular spells that check the caster level. If the class level is checked instead, or if feats make use of the caster level, then level drain will not have the caster level penalty. Spell effects created by a creature under level drain are just as difficult to dispel as from a creature without level drain. Level drain can be removed with restoration (or greater), but not with lesser restoration.