The obligatory -style intro:
The priest is perhaps the best healing class in the game with a surprisingly strong damage potential as well (should the player choose to enhance this aspect of the class). Their ability to heal, avoid aggro, dispel buffs on enemies, dispel debuffs on their allies, and Resurrect fallen comrades makes them an important member of any group, whether it's PvP or PvE.
A quick summary of the main things a Priest brings to a group:
* Healing: Cast a large and versatile collection of healing spells (heal-over-time, low-threat heals, fast heals, strong heals, preemptive healing, etc.)
* Protecting a target with Power Word: Shield
* Increasing a player's stamina with the Fortitude buff (on individual players and full groups)
* Increasing resistance to shadow damage with a buff (individual and group versions)
* Increasing a player's spirit, increasing health and mana regeneration (talent-based buff)
* Controlling their own threat through talents
* Dispelling both negative spells on allies and positive spells on enemies
* Resurrecting other players (out of combat)
While Holy Priests are not as effective in damage output, they are argued to be one of the best healing classes in the game, however, this is still debated.
Their primary stats are Spirit for mana regeneration, Intellect for a large mana pool and crits, and Stamina for more hitpoints. Stats obtained from gear, such as Mana points/5 seconds and +damage/healing are also important.
 Shadow Priests
Traditionally, the Shadow tree has provided a very good way to level the priest class, especially after level 40. Via talents and a targeted Spirit and Intellect build, Shadow Priests can have high, sustained DPS and incredibly high survivability with little downtime.
With the changes to the Shadow Tree in 2.0 Shadow Priests are now considered a much more viable class for end-game raiding, acting as mana batteries, for example, with Vampiric Touch and using their other spells to better the efficiency of the raid. Although Shadow Priests lack AoE and natural class abilities to retain mana, this can be alleviated with the use of consumables and the active use of Vampiric Touch.
Shadow Priests can:
* Spec for 25% threat reduction of their shadow spells
* Have a highly mana efficient spell in Mind Flay, increase warlock DPS through the use of shadow vulnerability, and provide good raid flexibility as a class capable of healing and DPS as the situation requires.
* Allow for up to a 45% increase in damage output, which is calculated after any and all bonuses from gear like all spell-casting classes. The only exception to this rule is healing spells.
Talented Building (Updated 11/13/08)