The game takes place in the Guild Wars fantasy world of Tyria, 250 years after the players defeated the Great Destroyer in the "Eye of the North" expansion. Five elder dragons sleeping beneath the continent have awoken, causing widespread destruction to Tyria and corrupting its inhabitants.
Guild Wars 2 brings the storytelling of RPGs into the MMO realm, and has a story filled with choices that allow you to play through the game many times and see different story lines and twists every time.
"Quests" as you know them are gone, and instead there are "events". These events happens in real-time in the game, and you can join up as they happen. Depending on the outcome, the world will change accordingly, and a new event will be triggered from that.
Any person you meet in Tyria is a potential ally, as you no longer need to join a group with someone to play with them. You will also receive the same rewards, when you play several people together, as you would, if you were playing all alone. So... - no one can steal kills, or loot from you anymore!