These descriptions are taken from Wikipedia.
Squidward Q. Tentacles (Rodger Bumpass) is SpongeBob and Patrick's effete cephalopod neighbor. Squidward has been referred to as a squid by the series creators, but he referred himself as an octopus once. He lives in a house shaped like an Easter Island Head. He loves art, and makes every one of his artworks have the subject of himself. He works as a cashier at the Krusty Krab, a job he dislikes. SpongeBob and Patrick consider Squidward their friend, but the feeling is, by far, not mutual.
SpongeBob SquarePants (Tom Kenny) is the title character of the series. He is an optimistic, energetic, lively sea sponge who physically resembles an artificial, kitchen-type sponge and lives in a pineapple-shaped house. He is often shown in the series working at The Krusty Krab as a fry cook. SpongeBob's hobbies in the show include practicing karate with Sandy, catching jellyfish and blowing intricate bubbles. He attends Mrs. Puff's Boating School, but has never passed; his lack of a driver's license serves as a running gag throughout the series.
Patrick Star (Bill Fagerbakke) is a pink, lazy, overweight, and dimwitted seastar who is SpongeBob's best friend. He lives under a hinged barnacle and his most prominent character trait is his low intelligence. Depicted as a negative influence on SpongeBob, he encourages dangerous and foolish activities that get the two into trouble. While typically unemployed throughout the course of the series, Patrick holds various short-term jobs as the storyline of each episode requires.
It should also be noted that some people believe that Patrick is, at least mildly, autistic.