A form is a character class function that enables or disables certain abilities as well as boosts class performance. Forms most commonly refer to Druid shapeshifting forms. In macros (specifically macro conditionals) and some action bar add-ons, this refers to a variety of forms that classes can adopt, such as 
[Stealth], 
[Shadowform], and 
[Ghost Wolf]. "Form" is synonymous with "stance" in macros.
Macro form numbers
| Form | Abilities | Form | Abilities |
|---|---|---|---|
| 0 | No form | 2 | Druid:  [Cat Form]Monk:  [Stance of the Fierce Tiger]Rogue:  [Vanish]Warrior:  [Defensive Stance]
|
| 1 | Druid:  [Bear Form]Priest: [Shadowform]Priest:  [Spirit of Redemption]Monk:  [Stance of the Sturdy Ox]Monk:  [Stance of the Wise Serpent]Rogue: [Stealth]Shaman: [Ghost Wolf]Warlock:  [Metamorphosis]Warrior:  [Battle Stance]
| ||
| 3 | Druid:  [Travel Form]Rogue:  [Shadow Dance]Warrior:  [Berserker Stance]
| ||
| 4 | Druid:  [Moonkin Form]Druid:  [Incarnation: Tree of Life]
|
Note that a Druid can only have Moonkin Form or Tree of Life. Restoration Druids using Incarnation will turn into a Tree of Life, while Balance Druids learn Moonkin Form automatically.
[Stance of the Sturdy Ox] and [Stance of the Wise Serpent] are only available to Brewmaster or Mistweaver respectively and occupy the first position on the stance bar.
Notes
- Warriors previously relied on a version of forms called stances, which restricted ability access much like Druid forms. These stances now act as buffs and have no effect on what abilities can be used.
Patch changes
Patch 6.0.2 (2014-10-14): [Travel Form] now automatically transitions between Aquatic, Land, and Flight forms depending on the Druid's location. 
Patch 5.0.4 (2012-08-28): Warrior stances no longer restrict ability access, and combat buffs changed.