From Diablo Wiki
Abilities are talents possessed by all characters in the game. Unlike skills, abilities do not require mana or other resources to use, and are not found on the skill trees. Abilities are basic functions, but they differ from the simplest character abilities like "walk" or "pick up item" since they require more than just a mouse click; an icon must be selected to enable their function.
This listing of abilities is speculative. Some are included since they were present in Diablo 2, others because they have been mentioned in various previews or seen in convention demos. The final list of abilities will not be known until shortly before Diablo 3's release.
(For instance,was an ability possessed by all Diablo 2 characters during most of the game's development, until it was removed just before the beta test began.)