"Planet Hulk" is an adaptation of a storyline from the "Incredible Hulk" comic book, in which Hulk is exiled to outer space by a bunch of other super-heroes, because he is such a menace on Earth. Of course, things don't go as the heroes plan, and, due to a wormhole in space, instead of being sent to the idyllic, tranquil planet that the heroes intended, Hulk is instead sent to Sakar, a divided world, ravaged by hostile monsters, and ruled by the cruel monarch known as the Red King.
The "Planet Hulk" storyline from the comics was huge, and there was no real way to adapt the entire thing into a movie, as a result, you get a much scaled down version of the story. There is a depth to the comic book which you just don't get in the movie, but the movie never feels rushed, or overly edited, so it scales down the comic book well. The two biggest changes from the comic to the movie are: 1) The Brood character is not present as a member of the Warbound, and 2) Silver Surfer is replaced by Beta Ray Bill as one of the gladiators that Hulk fights. In watching the "Making of" segment, apparently they didn't have the legal rights to use the Silver Surfer (possibly tied up with the company that currently is controlling the rights to the Fantastic Four), so they used Beta Ray Bill instead. He was a good choice, but he (basically an alien version of Thor) is kind of a silly character, who wouldn't be known by anyone but real hardcore Marvel Comics fans.
The movie goes right up to the point where the Red King is defeated. People who have read "Planet Hulk" know that this is not where the storyline ends. Again, the creators explain this in the "Making of" segment, saying that they wanted to make the film self-contained. The end of the comic, ties directly in to "World War Hulk" the next Hulk storyline. Also, the comic ends on a much more depressing note.
All in all, I enjoyed the movie, but really hope they make a sequel that adapts "World War Hulk."