A debuff is a passive, mostly temporary, detraction of a character. Anything which decreases elements such as attributes, causes decreases in total health and mana pool or regeneration, or direct decreases in combat ability through physical or magical attack power, is considered a debuff.
Debuffs are sometimes categorized based on how the debuff is gained:
Items - some items will grant temporary debuffs when consumed, in addition to buffs.
Active Skills - some active skills will bestow temporary debuffs, to either just the one target or to all targets within a certain range of the effect being used.