G-canon is absolute canon; the movies (their most recent release), the scripts, the novelizations of the movies, the radio plays, and any statements by George Lucas himself. G-canon always overrides the lower levels of canon when there is a contradiction. Within G-canon, many fans follow an unofficial progression of canonicity where the movies are the highest canon, followed by the scripts, the novelizations, and then the radio plays.
C-canon is primarily comprised of elements from the Expanded Universe including books, comics, and games bearing the label of Star Wars. Games and RPG sourcebooks are a special case; the stories and general background information are themselves fully C-canon, but the other elements such as character/item statistics and gameplay are, with few exceptions, N-canon.
S-canon is "secondary" canon; the story itself is considered non-continuity, but the non-contradicting elements are still a canon part of the Star Wars universe. This includes things like the popular online roleplaying game Star Wars: Galaxies and certain elements of a few N-canon stories.
N-canon is "non-canon". What-if stories (such as stories published under the Star Wars: Infinities label), game stats, and anything else directly contradicted by higher canon ends up here. N-canon is the only level that is not considered canon by Lucasfilm.