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 buffs like 
[Arcane Brilliance]) or those that are dependent on 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
Some buffs were introduced in Dragonflight. These include:
- Attack Power - Warrior,
[Battle Shout] - Intellect - Mage,
[Arcane Intellect] - Stamina - Priest,
[Power Word: Fortitude] - Movement Ability - Evoker,
[Blessing of the Bronze] - Versatility - Druid,
[Mark of the Wild].
Monks and Demon Hunters bring Raid Debuffs against all targets they hit, in the form of 
[Mystic Touch] (increased physical damage taken) and 
[Chaos Brand] (increased magical damage taken).
Warlords of Draenor
There were 9 standard raid buffs, which are provided by a number of different abilities. Duplicate abilities with the same effect would not stack, but you could combine buffs with different effects. For example, [Mark of the Wild]'s increased stats did not stack with 
[Legacy of the Emperor] as both increased stats; but [Mark of the Wild] did stack with [Power Word: Fortitude], as stats are a different buff effect to stamina.
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. While not technically raid buffs, 
[Mortal Wounds], combat resurrection, and Burst Haste are often just as desirable.
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.
Weaker versions of some buffs 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
- See also: Bloodlust effect
Another factor to consider when devising a group composition is a Burst of Haste. A cooldown that provides 30% Haste while it is up, it is different to the other buffs because it is temporary with a duration of 40s, and then cools down for 10 minutes from the start of the buff. These spells are 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:
- Evoker:
[Fury of the Aspects] - Mage:
[Time Warp] - Shaman:
[Bloodlust] (Horde) or 
[Heroism] (Alliance) - Hunter:
[Primal Rage]
- Prior to Battle for Azeroth, Hunters used the abilities [Ancient Hysteria] (Beast Mastery only) or
[Netherwinds] instead.
- Prior to Battle for Azeroth, Hunters used the abilities
If one of these classes is not available, a weaker version of the Burst Haste buff can usually be provided by a Leatherworking consumable. At the time of Dragonflight, this is specifically 
[Feral Hide Drums] which provides 15% Haste at the same duration.
Since Shadowlands, all Priests now also bring the 
[Power Infusion] cooldown which can be cast on a single ally to grant them 25% Haste for 20 seconds, although due to its smaller nature this is generally not considered as important as the full Burst of Haste type effect. Power Infusion stacks with the Burst of Haste effect.
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 
[60R] 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.
Patch changes
Patch 7.0.3 (2016-07-19): Raid buffs removed.
