The team we know we will see on the screen will be a 5-member squad composed of a pan-galactic group of loners and one-offs who will come together, apparently under some very shady circumstances, and, eventually, become a unit that will save the galactic day. Those members, are left to right, Drax the Destroyer, Gamora, Peter Quill, aka Star-Lord, Groot and Rocket Raccoon. Let's do a little exploring into each of these unique characters.
THE number one thing those unfamiliar with Guardians of the Galaxy need to know about Groot is that he is Groot. "Say what?" you say...yep I say. Expect that for the duration of his on screen time the only words you will hear Groot say are: "I am Groot." And as much as I look forward to that moment it isn't Groot's ability to talk the Guardians out of a jam that makes him a valuable member of the team. Groot is one mean motor scooter. Though I will confess to being unfamiliar with the earliest entries of Groot into the pages of the comics, it is his role in the Guardians of the Galaxy comics that will likely feed most directly into what we see on screen.
Groot, as each of these Guardians are, is the last of his kind. The former king of Planet X, Groot is a sentient tree-looking being with the ability to respawn from apparent destruction as long as the tiniest part of his body is found and cared for. As the former king of an entire planet, Groot was educated well and knows much more than he is able to communicate with those around him because of their inability to understand what he is saying. Safe to say "I am Groot" is to Groot as "memememememe" is to Beaker. Groot is a genius and it is put to use but so are his abilities to absorb the wood around him to grow in size and strength and do some hardcore damage.
Groot's entrance into modern Marvel comics came during the events of the Phalanx attempt to overtake the Kree empire in Annihilation: Conquest. We meet Groot, alongside several other future members of the Guardians, in a Kree prison where he is chosen to be a part of a low-tech team, led by Star-Lord, tasked with stopping the Phalanx. Groot immediately forms a nice bond with Rocket Raccoon and the team goes to battle.
Despite an apparent death or two, Groot survives, is regrown and used to help defeat the Phalanx and, along with Star-Lord and the other members of their team, moves on to become the Guardians of the Galaxy.
Just what route James Gunn takes Groot in the film next August will be interesting. It's easy to squeeze him in as comic relief and muscle but I'll hope we do get some depiction of how powerful and intelligent Groot is. His relationship to Rocket is a great dynamic and we know Gunn has said that the film only works if Rocket works so perhaps there's some room for Groot to grow. See what I did there? Though not officially confirmed, Vin Diesel will be the voice of Groot and he has said he will have just one line. We can all figure that one out.