If you've ever tried Elsword, you'd have the very basic idea.
An AP character uses an AP bar as opposed to an MP character using an MP bar. AP characters cannot charge AP by attacking, but they gain AP over time.
An AP character's skills and special abilities are decided via Skill Tree. You can purchase skills and passives with SP and equip them.
AP characters have only five slots to put skills in and you use the ASDFG keys to activate the skills. i.e. you have Gravity Ball in slot A, so you press A to activate it.