Stupid question , I know what PvP is

but what is WvW and how does it work?
WvW = "World Versus World"
It is an absolutely enormous, four-section map with three teams playing on it. Three teams each control (at the start) one homeland map, and in the middle is the Eternal Battlegrounds. The more of the map you control for your world, the more PvE bonuses everyone else gets "back home." You conquer castles, depots, guard dolyak caravans, bombard walls, rebuild boiling oil pots above your gates, etc etc, making your way across the map in a vaguely
Onslaught-like gametype.
If you win the map (takes a long time), your world's PvP rating goes up and you find yourself against other, higher-rated worlds next time.
Who is on your team?
EVERYONE. Anyone from your shard (Moledune, Sands of Kryta, Sea of Sorrows, etc) who wants to can join in. You can literally have 200v200 battles, theoretically 500v500, etc. I've seen groups of 50+ attacking the gates of a castle while defenders try to fend us off from the battlements. It is hectic, chaotic, free-flowing, and
downright awesome. In practicality - at least in the beta - I've never seen more than maybe 150 people in the same battle. That's still
absolutely insane and will lag your computer into the ground, but my goodness is AoE fun in a group that size. The only thing more fun than attacking a castle with a group that large is being the guy on the battlements with the CANNON.

Your "level" remains what it is, in terms of skills, weapons, armor, etc, but you are equalized to level 80 for level and HP purposes. The tradeoff for this is that you still get drops and experience for playing, and a darned lot of it at that. It is a very legitimate way of leveling up your character.
True story: I got an entire group on Moledune to attack an outpost while screaming "FOR WAFFLES!" as their battle-cry. My life is now complete.
My opinion is that WvW was an amazing, wonderfully refreshing style of PvP that has been long missing in MMOs. It is not without its problems - particularly timezone issues affect the start of the game right now - but it is a BEAUTIFUL thing when it gets rolling. I like it better than anything else in GW2 so far.