<Ability>: it's like a skill, but it doesn't require mana to activate and is usually only a boosted version of something the owner can already do, EX: counter ---> Instant Counter.
|Skill|: allows the user of the skill to perform an action with the help of mana, EX: Iai Slash ---> Iai Spirit Slash.
{Spell}: a form of skill that relies completely on mana, EX: Fireball, Invisibility, ect.
Abilities can be things like being able to see further, reacting faster, and being able to see in the dark.
Active Passives: a form of passive that be turned on or off, EX: Oppressive Aura.
True Passives: a form of passive that cannot be turned off, and is constantly on, EX: Regeneration
Abilities also sometimes have drawbacks/weaknesses, EX: Night vision --> glowing eye(s).
Skills are more damaging than spells, but spells are generally stronger than skills at range.
A skill can also be performed without the use of mana, but then it's basically just an action, EX: Skill: Dash = Action: really quick running.
Passives of any kind can be temporarily/permanently disabled by certain skills, spells, or items, but never by abilities. And on the topic on disabling, some abilities can be disabled as well but only ever temporarily.