A buff is a change to one or more of the normal parameters controlling game play. A buff can, for example, increase your troops’ attack strength by 20 percent for 12 hours. Another can prevent your city from being scouted for a length of time.
Some buffs are found as rewards in the game while others must be purchased using diamonds in the game store. That tab is found under the briefcase icon in the bottom left center. Some buffs can also be stocked in the alliance store.
Buffs generally expire. Skills and researched technology, however, do not.