There are three types of Abilities, Action, Reaction, and Support. Here is the description of each type of Abilities:
These are job-specific abilities which players can utilize during battle. They require a certain amount of Soul Jewels to use. The 1st Action ability slot is restricted to the unit's current job.
These are passive abilities that give bonuses to a specific unit. You can set up to 2 different Support Abilities per unit. There are some Support Abilities which can benefit specific Jobs. (Example: Hawk Eye: Range +1 -> Affects only Ranged-type jobs)