Fayth - Fayth is the Priest's primary offensive skillset and it consists mainly of balance-based attacks. Your seraph is also controlled through this skillset, allowing you to sap the mana of your enemies and absolve them to instantly kill hem at 50% mana. This is the skillset used for PvE.
Devotion - Devotion is the Priest's secondary skillset, and it consists of combat-oriented rites, passive defenses and active healing. The skillset is useful in both PvP and PvE.
Kanai - Kanai is the Priest's fully offensive skillset, besides prayers it has no PvE abilities. The Priest gains Fanatism on attacking their target which they can then use in many different offensive abilities or to enhance the main attacks from Fayth like Smite.