Moviefone has the full breakdown for all the non-comic geeks in the audience.
Who are the Guardians of the Galaxy? Think of them as "space cops." They are a largely extraterrestrial band of cosmic superheroes who fight against intergalactic threats. They operate out of Knowhere, an observational space station located at the end of the universe.
There was an earlier version of the team that debuted in 1969, but their backstory (they traveled from an alternate future to team up with the Avengers today!) makes them a little too impenetrable for casual audiences. This movie will likely focus on the 2008 rebooted version, a grittier update that revitalized several C-list Marvel characters.
Who Is On The Team? Star-Lord, the human leader. A Han Solo-type.
Nova, a young brash human, who inadvertently inherits a cosmic protector job from a dying alien; in short, he's Marvel version of the Green Lantern. Nova isn't technically a member of the Guardians, but Marvel is making a big marketing push to turn him into the "Spider-Man of outer space."
Drax the Destroyer, a human mind placed into the body a big green ultimate gladiator body. He has been programmed with the sole objective of "kill Thanos." He's died more than once trying to achieve this goal. (He usually gets better.)
Gamora, adopted, alien daughter of Thanos; her hand-to-hand combat skills make her the deadliest woman in the Marvel universe. Usually the love interest to the main hero.
Adam Warlock, artificially-created human who can wrap himself in a cocoon to become stronger. Has a tendency to turn into a cosmic god of David Bowie-proportions.
Rocket Raccoon, a wise-cracking alien raccoon that loves to fire guns. It's as stupid as it sounds, but somehow it works.
Groot, a giant, walking alien tree, like the Ents from "Lord of the Rings." Best buds with Rocket Raccoon.
Then there are other members: Cosmo (long-lost dog sent into space by Cosmonauts, who gained ability to talk), Moondragon (bald-headed lady telepath), Mantis (celestial martial artist/precog), Bug (insectoid warrior from a microscopic dimension), Jack Flag (Captain American-inspired vigilante who is a fugitive on Earth) and Quasar (male and female cosmic protectors) -- but don't expect anything more than a wink and a nod to them.
Who is the Villain? It's got to be Thanos, that purple-faced hulking bad guy seen in the last few moments of "The Avengers." Thanos is from Titan, the moon of Saturn. The Titans are a lot like the Asgardians in "Thor," godlike and mythological, but technically an alien race. Thanos was born a hideous freak -- making him something of a mutant among the Titans -- and he soon falls in love with Death. Not the idea of Death, but the actual physical embodiment of Death. In the Marvel Universe, Death is a lady grim reaper who travels the universe appearing whenever life is lost; Thanos falls madly in love and tries to bring about carnage to impress her. And what better way to impress her than killing off the whole galaxy?
What Is the Movie About? You think Marvel is telling us? Our best guess is that Thanos tries to bring about the end of the universe and it's up to the Guardians to stop him. Expect "Guardians" to lead into "The Avengers 2," where Iron Man, Thor and Cap will have to deal with an Earth-bound Thanos.
What Should I Read? Well... the stories set in the Marvel Cosmic Universe can get pretty in-depth. The movie will probably take most of its plot from "The Thanos Imperative" by Dan Abnet and Andy Lanning. In itself, it's a perfect summer blockbuster story, but it's also a culmination of a "Star Wars"-like space opera, that began back in "Annihilation."
If you're feeling truly audacious, and want to get the whole epic story, then here's the complete reading list in chronological order: "Annihilation, Volumes 1 - 3" "Annihilation: Conquest, Volumes 1 and 2" "Guardians of the Galaxy - Volume 1: Legacy" "Guardians of the Galaxy - Volume 2: War of Kings, Book 1" "Guardians of the Galaxy - Volume 3: War of Kings, Book 2" "War of Kings" "Guardians of the Galaxy - Volume 4: Realm of Kings" "Realm of Kings" "The Thanos Imperative" "Annihilators" "Annihilators: Earthfall"