Each of the heroes initially has 2 or 3 special abilities. In most cases, it's one active and one passive skill. Active skill must be activated each time, spending one of a few willpower points. A passive skill does not require your action, e.g. automatically dealing damage to enemies nearby or being able to run across fields occupied by allies.
Use your special abilities on a regular basis, as long as you have enough willpower. You can always choose whether you want to use the basic or special version of a skill (it requires more willpower.) When you max out all of your character's stats, you will be able to select a new skill under each of them. Aside from being active or passive, they can be offensive/defensive/supportive.