Set in space, Guardians will chronicle the adventures of five mismatched 'heroes' who get into conflicts with evil forces etc. The movie will be more tangentially connected to the Marvel Cinematic Universe than their other films (i.e. don't expect Iron Man to show up halfway through) but will partly set things up for Avengers: Age of Ultron, which will follow in 2015.
I am not acquainted with the Guardians as a superhero team, although I did thoroughly enjoy the Rocket Raccoon solo strip in the 1980s (when it ran as the back-up strip in the Marvel UK Transformers comic), which was enough to make me want to check this out. The trailer is also pretty cool. It's good to see Marvel embracing full SF craziness for once.