Most often, a buff refers to a temporary beneficial spell or effect placed on a player. Buffs can range in effect from simple personal shields to buffs which augment almost every category of stats a given player may have for their race or class.
Buffs can be of two types: those that expire over time (simple buff like 
[Arcane Brilliance]) and those that are dependent of the environment around you (like the AoE effect given by totems).
You can dismiss an active buff that expires over time by right-clicking on its icon.
Most buffs do not stack with other similar buffs. One prominent exception is heal over time spells, heal over time spells from multiple characters and different heal over time spells from the same character (such as a druid's 
[Regrowth] and 
[Rejuvenation]) will stack with each other.
Buff can also be used as a verb, i.e. "to buff" means to apply buffs.
Buff is also used to represent a change to a class or ability that makes it more effective or useful. It is the opposite of a nerf.
Raid Buffs
There are 9 standard raid buffs, which are provided by a number of different abilities. Duplicate effects do not stack, but you can have multiple buffs. For example, 
[Mark of the Wild]'s increased stats do not stack with 
[Legacy of the Emperor], but [Mark of the Wild] does stack with 
[Power Word: Fortitude].
Some abilities apply more than one buff. When you are in the Proving Grounds, you automatically receive all 9 buffs until you leave the instance. A raid that consists of a party capable of providing all 9 buffs is considered an optimal raid, and when forming a raid party you should try to ensure that each of the buffs will be available.
The buffs are:
- Stats: Increases Strength, Agility, and Intellect by 5%.
- Stamina: Increases Stamina by 10%.
- Attack Power: Increases attack power by 10%.
- Spell Power: Increases spell power by 10%.
- Versatility: Increases Versatility by 3%.
- Haste: Increases Haste by 5%.
- Critical Strike: Increases Critical Strike by 5%.
- Multistrike: Increases Multistrike by 5%.
- Mastery: Increases Mastery by 550.
The following tables show which classes and specs are capable of providing which buffs:
Weaker versions of some of these buff can be provided by consumable items. While these versions are weaker, it is worthwhile to have an inventory of these items when raiding, in case a class capable of providing the full version of a particular buff is not available. Like the abilities above, these do not stack with similar effects:
- Stats:
[Drums of Forgotten Kings] - Stamina: [Runescroll of Fortitude III]
Burst Haste
In addition to the 9 standard raid buffs, there is a tenth buff that it is important to have available in your party. Unlike the 9 buffs listed above, this buff--Burst Haste-- is temporary and is generally used at a critical moment during a raid encounter predetermined by the raid leader based on the party's strategy.
This buff can provided by the following classes:
- Mage: [Time Warp]
- Shaman: [Bloodlust] (Horde) or [Heroism] (Alliance)
- Hunter: [Ancient Hysteria] (Beast Mastery only) or [Netherwinds]
If one of these classes is not available, a weaker version of the Burst Haste buff can be provided by 
[Drums of Rage].
List of notable buffs from environment
Lordaeron's Blessing
Increases stamina by 5%.
Lasts: 30 min
How to get: Take control of the northpass tower in Eastern Plaguelands, click the altar that appears outside the door.
Notes: Persists in Scholomance and Stratholme, and persisted in the original Naxxramas.
Fire Resist
Increases fire resistance by 83.
Lasts: 1 hour
How to get: 
[Mind Control] (either by the Priest spell or the Engineering device) a Scarshield Spellbinder, found in Blackrock Spire.
Notes: Sometimes the tooltip will say differing amounts of Fire Resistance, but the buff always provides 83 resistance. Stacks with Paladin's 
[Resistance Aura].
Rallying Cry of the Dragonslayer
Increases critical chance of spells by 10%, melee and ranged by 5% and grants 140 attack power, for characters below level 61.
Lasts: 2 hours
How to get: When the head of Onyxia or Nefarian is returned the players located in (or nearby) Orgrimmar or Stormwind are buffed with this buff.
Spirit of Zandalar
Increases all stats for a value depending on level for 2 hours.
(Old Effect: Increases movement speed by 10% and all stats by 15% for 2 hours.)
Duration: 2 hours (As of 1.11, this buff no longer persists through death)
How to get: When the 
[Heart of Hakkar] is turned in to Molthor on Yojamba Isle as part of The Heart of Hakkar quest, players on the Isle and in Booty Bay will receive the buff.
Warchief's Blessing
Blessing of the Warchief: Increases health by 300. 15% haste to melee attacks. 10 mana regen per tick.
Lasts: 1 hour
How to get: When the head of Rend Blackhand is returned after a player completes "For the Horde!", the end of the first part of the Onyxia prequest chain, all players located in Orgrimmar are buffed with this buff.
Songflower Serenade
Increases critical strike rating and spell critical strike rating by 70 and all attributes by 15.
Lasts: 1 hour
How to get: Cleanse a Corrupted Songflower in Felwood.
See also
External links
nl:Buff