Shadow Priest vs. Holy Priest
When we discuss Priest roles, we typically use the terms “Shadow Priest” and “Holy Priest” to describe the two Priest class paths. This is somewhat misleading however, since each path optimally includes talents within the Discipline tree. Therefore, “Shadow Priest” actually refers to those with Shadow/Discipline builds and Holy Priest refers to those with Holy/Discipline builds. Both are valid builds, and both have a role to play, yet however similar they are in some ways, they are also very different.
Shadow Priest
The Shadow Priest puts the majority of their points into Shadow, usually at least 31 points so they can get Shadowform. This is the preferred build for leveling to 60 as your damage output is vastly increased, rivaling most other classes for pure dps. In Shadowform, you cannot heal, so you are focused on damage, with some added utility spells like Silence. In instances, the Shadow Priest can still heal, and be the primary healer most of the time, all the way up through Upper Black Rock Spire (UBRS), assuming you have a backup healer. However, you will have less mana, heal for less hp per heal, and spend more mana casting heals than the Holy Priest. Shadow spec is what you want if you want to spend the majority of your time in PvP.
Holy Priest
The Holy Priest is the best healer in the game. He will be able to heal more hit points on a single load of mana than any other class. Your typical Holy Priest will have at least 30 points in Holy, focusing on the talents that improve his healing type spells. As a Holy Priest, your damage is lackluster compared to Shadow, so you will not spend your time out soloing mobs all day, and if you do, it will not be as productive. However, in high-end, raid-style instances, you will be well suited to be in group one as the primary healer. You can spec Holy while leveling, but I advise against it, because as a Shadow Priest, you can reach level 60 much faster, then for a small fee respec to Holy. After reaching 60, if you intend to spend the majority of your time as a primary healer in instances and on raids in places like UBRS and Molten Core (MC), then you want to permanently spec as Holy/Discipline.