From Wookipedia.

Retroactive Continuity (i.e., how we explain away all those nasty contradictions in Star Wars): http://starwars.wikia.com/wiki/Retcon

"Official Levels of Canon" (I'm not sure how official they mean when they say official, though):
http://starwars.wikia.com/wiki/EU#Official_levels_of_canon

Keep following links and you'll find lots of fascinating stuff that'll distract you from studying for hours. :D