Buffs

 are an element first added to Terraria in patch 1.0.5. They are temporary bonuses that increase the player's stats, or sometimes even grant special abilities such as walking on water. Buffs require certain items to activate, and, once activated, they will appear in the buffs bar in the upper-left corner of the screen.

A buff can be forcefully deactivated by right-clicking on its icon. Buffs of the same type do not stack--reactivating a buff will merely reset its duration. Also, only 10 buffs may be active at a single time. Activating an 11th buff will cause the one furthest to the left in the buffs bar to be instantly cancelled, and the new buff will effectively take its place.

Bugs

 * The Campfire and Heart lamp buffs are seen as debuffs to the nurse, and will cost an extra  when healing near a campfire or Heart Lantern.

Trivia
Even though the Battle Potion and the Water Candle have similar functions, the former is a Buff and the latter is a Debuff.