There are several different attributes into which Ability Points can be invested in. These each add various stats onto your character. These can be previewed and reset before pressing the Apply button. Once you have pressed the Apply button, however, the Ability Points cannot be reset.
Each level grants you 3 Ability Points (4 for the Extreme class), which can be invested into POW, DEX, INT, VIT or STM. Each will affect your stats in a different way.
(Unverified) Each class may have its own individual growth rates.
Main Articles: Archer Growth Rate