Canon is what Wizards defines in their current campaign setting. But they have a whole lot of contradictions that are supposed to be canon. The DM can make up his own rules but if you take the books, raw, no alterations, that's what's "canon".And what is canon? What was written in 1e, or is it what was written in 3e, or is it what has been written in 4e? No wait, it is what the DM makes up at the table.