Eric Tolle said:
From what I understand, there aren't "levels"- canon is binary. If it was in the movies its canon, if not, then its not.
This simplifies things because one can avoid 99% of the idiocy in the Star Wars universe by simply saying "canon only". Since I've yet to see anything in the EU that's worth including, there's no real loss.
For Star Wars, Lucas Licensing(thanks Faraer) does have levels set up. Essentially, there are 4 levels.
G-Canon is the top tier, and overwrites everything that contradicts it. Stands for George Lucas, and its everything from the movies and anything else directly from Lucas.
Next down is C-Canon, which is 'continuity canon'. This is (almost) everything from the Expanded Universe. Books, novels, games, comics, what have you. Some C-canon has been bumped up to G-canon, also, like Coruscant, Quinlan Vos, and some other similar things. (Note that Lucas himself felt these were worth including, for those that throw outright hatred at the EU

)
Next is S-canon, for 'secondary canon'. Basically this is the old stuff that is mostly ignored these days. The Marvel Comics series, very old books, etc. Some of this does get bumped up to C-canon.
Lastly is N-canon, for 'non-canon'. Infinities comics, what-if stories, etc. These are pretty much the ONLY thing that are stated to NOT be canon. Everything else above it, however, IS. Just that G-canon trumped everything under it, and so on.
...I said it was complicated...