Well, first off, there are five of them:
Peter was abducted from Earth as a child... and became an interstellar adventurer.
He's also trying to get people to call him Star-Lord.
Groot's the walking, talking tree.
Although "talking" might be an overstatement — his communication skills are limited to this phrase:
He's got a personal vendetta against Ronan (the main bad guy).
In the trailer, he's called a maniac, and in the comics, he's called Drax the Destroyer, so yeah... he's a real sweetheart.
Gamora was raised by Thanos to be the perfect weapon... and his personal assassin.
Oh, and before you ask, Thanos is the purple guy from the end of The Avengers.
He enjoys shooting things...
Trouble is coming to save the galaxy.
All images courtesy Marvel.