Priests are the masters of healing and preservation, restoring their wounded allies, shielding them in battle, and even resurrecting their fallen comrades. While they have a variety of protective and enhancement spells to bolster their allies, priests can also wreak terrible vengeance on their enemies, using the powers of shadow or holy light to destroy them. They are a diverse and powerful class, highly desirable in any group, capable of fulfilling multiple roles. - WoW.com

Being a priest, I've found that Shadow spec. is better for leveling, Holy is better healing in raids and dungeons, and Discipline is better for surviving in pvp. With the new dual spec'ing, you could be primary spec. Holy and secondary as Shadow. This way you could either dps or heal.

Armor sets that you should try to get when you lvl80 are:
Holy & Shadow

This is what I use for pvp. It works pretty well in battle grounds and arenas.

And this is what I've been using for healing in raids and heroics.

-Syles