Auto-cast is a toggle that allows a spell or ability to be used automatically. When auto-cast is active for an ability, it's signified by yellow sparkles rotating around the edges of the ability's icon. Auto-cast can be activated or deactivated by right-clicking on an auto-castable ability.
In Warcraft III
If an ability in Warcraft III can be auto-cast, its icon will have a yellow bracket in each corner:
In World of Warcraft
If an ability in World of Warcraft can be auto-cast, its icon will have a small yellow triangle in each corner: