Canon is what happens on the screen. So it's canon that Giles has said, for example, that a vampire is not a person. It's NOT canon that Giles was either (1) right or (2) telling the truth. The Buffyverse canon is still open, which means that Joss and his writing staff get to reinvent and reinterpret and retcon canon at will - this happens all the time. They've made many changes in the mythology (for one tiny example, Spike's age). In the Giles example, he could (1) change his mind, (2) reveal that he was spouting CoW propaganda to ease Buffy's mind, (3) learn something new, (4) start telling lies the other way if he had been telling the truth previously, or (5) many other things we haven't thought of yet.
The "real" source material for a fandom. For a movie, this would be the plotline of the actual movie. For RPS, this would be the lives of the actual individuals as determined by publicly-available material such as interviews, television appearances, etc. Examples of AFI canon elements are: AFI is a band, the members of AFI are named Davey, Jade, Adam, and Hunter, Smith is AFI's tour manager, Davey has a stuffed chicken named Pecky, etc.
Any facts or information stemming from the episodes of a show. (See also "fanon.")