There will be two types of PvP available in Guild Wars 2; World PvP and Structured PvP.
World PvP will allow you to play characters of any level. It is intended to be a more relaxed bridging point between PvE and structured PvP. It will be on a larger scale than the original Guild Wars PvP with large battles which could take place over a long period with people coming and going.
RTS elements will be added into World PvP. The World PvP Maps bear a resemblance to RTS-maps and give you a chance to play different roles. You can conquer stone mines, which will then deliver stone by caravan to your catapults, you can attack and cut off caravans of the enemy, you can operate as a scout and inform the rest of hostile movement, you can man watchtowers and defend, or you can just run up to a lost PvP'er and teach him a lesson.
Each battle will have a number of different objectives which can be met such as taking guard towers or defending choke points or joining a larger battle to take a castle. Battles will take place in the Mists.
Victory in World PvP will confer benefits to your world. This may include improved drop rates, better energy regeneration or other bonuses.
Structured PvP is similar to today's GvG. Structured PvP-participants will have access to the same skills and gear, which ensures survival is indeed of the fittest (and most strategic), not the one that has spent the most time online. It will allow you to enter the game at maximum level with all skills, races, items and classes automatically unlocked.
Party size in World PvP has no limit, and you can charge into a limitless battle