Star Wars canonicity is broken up into five canon classes. The reason for this is that George Lucas does not consider any SW media aside from the movies to be part of his universe. So all other information is, to a degree, separate from his movies. Any and all Star Wars material unrelated to the movies is canon in what is known as the Expanded Universe (EU). I will draw these canon classes out here and describe the meaning of each class and what each class is comprised of (in my own words). I memorized the canon classes a long time ago, but to others who are not so well-versed in SW canon, understanding what is or is not canon or what canon class different SW medium falls under is crucial. I also just grow tired of explaining this in threads. So here they are.

G-Canon: George Lucas Canon. This is the highest canon class. Under this class is any and all material of or relating to the six Star Wars movies. As well, there are certain events, characters, or concepts that Lucas has acknowledged as being part of his universe, such as Darth Bane and the Rule of Two. However, despite Bane's existence in Lucas's Star Wars, his backstory or any major information regarding him, or any other character/concept not mentioned in the movies, is unknown. Now, there is some question concerning whether the novelizations of the movies are G-Canon. The technical defining parameters of G-Canon include all material associated with the movies, but in general, the movies are the only source material regarded as G-Canon.

T-Canon: Television Canon. This is the canon class that involves any televised Star Wars stories, such as the Clone Wars. Although George Lucas has had some involvement in various Star Wars shows, they are not technically part of his universe and are reserved for this class. T-Canon is the second highest canon class and is the highest ranking class made up of EU information.

C-Canon: Continuity Canon. This is the main EU canon class. Beside TV shows, C-Canon constitutes all EU material. This includes novels, comics, video games, audio dramas, etc. Because C-Canon has the broadest scope of material under it, this will be the canon class most often referred to when citing the EU and is, as a result, the most important canon class, next to G-Canon.

S-Canon: Secondary Canon. This is a class defined as ambiguously canon. It includes anything that has yet to be confirmed as canon or material that may or may not be canon anymore.

N-Canon: Non Canon. This is fairly straightforward. N-Canon pertains to anything in the Star Wars universe of any media form that has been specifically stated as non-canon. There are a select few things that have been established as being universally N-Canon, regardless of what they originated from. The prime example of this is game mechanics (character upgrades, semantics of fights, etc.). Although the story lines in Star Wars video games are canon (barring alternate endings), game mechanics are always N-Canon. N-Canon is typically referred to in Star Wars media as Infinities.

Leland Chee, the Keeper of the Holocron for Star Wars continuity, is the one who developed this system. He has personally attested to its validity and described it in his own blogs.

The database does indeed have a canon field for each individual entry and for sources, though the canon level of the entry would overide the canon level of the source since it factors in other sources associated with that entry. When determining canon levels for individual entries, anything in the films and from George Lucas (including unpublished internal notes that we might receive from him or from the film production department) is considered "G" canon. A new level we recently added is "T" canon, comprising of the theatrical release of The Clone Wars and the television series, in addition to the planned live-action television series. Next we have what we call continuity "C" canon which is pretty much everything else from the EU. There is a secondary "S" continuity classification used for older published materials created when there was less attention to making everything in the EU fit with everything else in the EU. But, if it is referenced in something else it becomes "C". Similarly, any "C" canon entry that makes it into the films can become "G" canon. Lastly there is non-continuity "N" which we rarely use except in the case of a blatant contradiction. Any contradictions that arise are dealt with on a case-by-case basis.

If there is any doubt that EU continuity is recognized as canon, those doubts should be summarily halted. George Lucas explained in the introduction to the novel Splinter of the Mind's Eye that he allowed for expounding on his works because he sees the potential for storytelling. Lucas himself has also been involved in the production of the novelizations of his movies, which tend to be released before the film itself. As well, Lucas has borrowed characters, locations, and events from the EU and used them in his movies (Coruscant, Quinlan Vos, etc.).

It was't long after I began writing Star Wars that I realized the story was more than a single film could hold. As the sage of Luke Skywalker and Jedi Knights unfolded, I began to see it as a tale that could take at least nine films to tell—three trilogies—and I realized, in making my way through the back story and after story, that I was really setting out to write the middle story.

After Star Wars was released, it became apparent that my story—however many films it took to tell—was only one of thousands that could be told about the characters who inhabit its galaxy. But these were not the stories that I was destined to tell. Instead they would spring from the imaginations of other writers, inspired by the glimpse of a galaxy that Star Wars provided. Today it is an amazing, if unexpected, legacy of Star Wars that so many gifted writers are contributing new stories to the Saga. This legacy began with Splinter of the Mind's Eye, published less than a year after the release of Star Wars. Written by Alan Dean Foster, a well-known and talented science-fiction author, Splinter was promoted as a "further adventure" of Luke Skywalker. It hit the bookstores just as I was preparing to write my own "further adventure" of Luke, in the form of a script entitled The Empire Strikes Back.

It seems only fitting, after all these years, that Splinter would be republished as I prepare once again to write another further adventure set a long time ago, in a galaxy far, far away...

--Taken from Splinter of the Mind's Eye

Further elaboration on Star Wars canonicity has been presented, as in Star Wars Galaxy #6, where a brief article describes the purpose of the Infinities logo and states that all stories without the Infinities logo are canon.