A strong believer in the light – almost zealous in their belief, causing them to feel no remorse about damaging enemies of the light. A Discipline priest is a battle healer, smiting down their enemies in order to bravely save their allies. They have adopted the traits of both healing and shadow priests in order to make sure their allies stay alive.
An extremely useful tool for overall optimization
WoWhead Discipline guide
http://www.wowhead.com/discipline-priest-guide
Icy Veins Discipline guide
https://www.icy-veins.com/wow/discipline-priest-pve-healing-guide
Noxxic Discipline guide