Discipline/HolyFollow

It's a PvP healbot spec. It relies on talents that increase personal survivability first, and increase healing ability and mana supplies second. Because the spec itself has precious little firepower, it's always used as a partner (or support) for a strong DPSer.

The offensive potential of the Disc/Holy priest is only partly in its nukes. Offensive Dispel casts are a big part of it, canceling not only standard buffs like Fortitude but also defensive spells like Shield and self-buffs like Arcane Power and Innervate. With Mass Dispel we can also bypass the "ultimate defenses" of Ice Block and paladin bubbles. The other large component is Improved Mana Burn, which can never be underestimated.

On its own, a disc/holy priest tries to maintain distance, keep itself healed, keep SW: Pain up, and toss out nukes and mana burns as the opportunity presents itself. Many victories don't come until the enemy is out of mana or close to it, particularly against other healers.

In groups, the priest is primarily a healer, with offensive abilties used on a "when you can manage it" basis. The big challenge for the priest is keeping himself alive-- which, against certain opponents, is harder than it sounds. When allowed to heal unmolested, the priest is able to keep his allies alive for an extended period, so enemies tend to target the priest with prejudice.

Key talents include Imp. Mana Burn, Focused Will, Healing Focus, Blessed Recovery, and Improved Shield. Mental Agility is important because of the reliance on insta-cast spells. Some other talents are debatable; I personally am a big fan of Reflective Shield for the surprisingly good offensive potential, but Enlightenment is viable too and right next door. Whether you pick up either of them is dependent on whether or not you also like Pain Suppression, which is a hotly debated talent itself.