In the Class system, each character chooses a specific class at character creation, a defining archetype which determines their fighting style for the rest of the game. Classes grant bonuses and determine equipment, but their most important role is granting abilities - and a lot of them!
Characters purchase abilities using AP from their class's grid, a network of connected abilities - you can purchase any ability connected to your starting node, or to any node connected to an ability you already possess. This expenditure can usually be freely made outside of combat, but depending on the campaign and its format, it might be restricted to between sessions or in periods of rest.
A level 1 character begins with 200 AP, and they earn more as their party defeats enemies. A character always has access to every ability they've learnt, forming a diverse portfolio of abilities.
Some support abilities have drawbacks (such as Turbo MP) or are mutually incompatible with one another (like Troubadour and Trance) - for this reason, or any other, a character may freely deactivate or reactivate any ability they have learned, as long as they are not currently in a battle. They might also desire to deactivate a Reaction ability to prevent it from being triggered, for example.
See this list for a key for icons on the grids.
Once there are better grids this note'll be removed, but Attribute boosters all cost 200 AP
Alchemist (ALC)
2/1/5/2/0 - 60/40
Archer (ARC)
5/2/3/0/0 - 60/40
Black-Mage (BLM)
0/0/0/5/5 - 0/100
Blue-Mage (BLU)
2/2/0/3/3 - 50/50
Gambler (GMB)
3/2/2/3/0 - 60/40
Lancer (LNC)
4/3/0/3/0 - 70/30
Ninja (NIN)
3/1/5/1/0 - 60/40
Paladin (PLD)
2/5/0/1/2 - 90/10
Red-Mage (RDM)
2/2/2/2/2 - 50/50
Songstress (SON)
2/1/4/2/1 - 60/40
Summoner (SMN)
0/0/0/6/4 - 0/100
Warrior (WAR)
4/2/4/0/0 - 70/30
White-Mage (WHM)
0/0/0/5/5 - 20/80