G4TV gives an awesome preview of some World vs. World action in the video above a long with a full article that gives you all the details of a full siege.
Anyone can join in on the WvW battle and several GW2 game designers have stated that WvW should act like a stepping stone for those players who are interested in learning about PvP but not quite ready to jump in to the competitive side of things.
As you enter in to World vs. World, regardless of what level you actually are, you’ll be scaled up to level 80. You’ll retain all of your skills, abilities, and armor, but it will all be scaled up to level 80. This is different than in competitive PvP where everyone is given the same gear, and all of your abilities and traits are unlocked.
What this boils down to is that if you’re a level two player fighting a level 60 player in WvW, the level 60 player will have a huge advantage over you. They’ll have a variety of skills and better gear than you do, even though you’ve both been scaled up to level 80 in the environment. In competitive PvP, everyone is on an even playing field, but that’s not the case in WvW.