Auto-cast
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
In Warcraft III, auto-cast was seen on a variety of abilities, such as Repair, Heal, Web, and Abolish Magic.
If an ability in Warcraft III can be auto-cast, its icon will have a yellow bracket in each corner:
In World of Warcraft
In World of Warcraft, auto-cast is generally seen on abilities used by hunter pets, minions, and other summoned units. This includes abilities like 
[Growl], 
[Deadly Bite], and 
[Devour Magic].
If an ability in World of Warcraft can be auto-cast, its icon will have a small yellow triangle in each corner:
External links
Warcraft III - Spell Basics (See "Autocast Spells" section)