What is canon? Canon is information that is assumed as true because it has been published. If a planet was blown up in a space opera novel, the author had better have a good explanation if it's opening up trade routes in the sequel. Even if you're on your first novel, you can pretend what you've written earlier in the novel is canon. This can help you if you're discovery writing or writing a thinly outlined scene, because there are somewhat limited possibilities for what characters can and will do. If you've established the fact that a character is highly arrogant, you don't have to scramble quite as much deciding what he'd do in given situations. However, Canon can also be a pain. If you made a mistake and had a nation that was conquered back in chapter three declare war in chapter twenty and you don't realize it until chapter twenty-five, well, you have a problem. (This example can be called a discontinuity error) Luckily, canon/discontinuity errors can be fixed if you're within a novel, and if you don't catch a series-spanning problem than your editor probably will. Keep track of your canon, and good luck with your writing.
Have you ever had an error of canon cripple your novel or found a discontinuity error in a book or series that you were reading? Leave a comment, I'd like to hear about it.