Hero Abilities (HA) are unique for each Hero and can be active (played like a spell or attachment) or passive. Do take note that active HAs are not treated as spells and will not trigger interactions like spells would.
There is one HA for each of our Heroes, although there may be more than one each at some point in the future.
Here is a good article by Just Add Bacon which mentions some of the Hero Abilities and how they can be maximized using specific cards belonging to the corresponding prisms!