Canon, for those who do not know, is what actually is ture about the storyline, as opposed to what is just theory.
For convenience, I will repeat what the official line is on what parts of the published storyline, in any medium, is canon.
The only indisputable canon is the films. What the films say cannot be contradicted, unless something is definitively decided to be a mistake, like the ROTJ rank badges. The Continuity and Production department at Lucasarts handles these issues.
After the films, the only other published material that is canonical are, in this order:
The screnplays
The novels
The radio plays
These are canon, Anything they say is considered to be true unless superceded by a source above them. However, it is worth noting that large amounts of the novels and radio plays have been rejected lately- Obi-Wan's brother being OwenLars, and Luke's sabre construction in the radio play being two good examples.
It is best to take these things as goodguidelines rather than fact.
Expanded Uiverse material is NOT canon. Therefore, anythuing in the EU has no bearing whatsoever on the films. Hence, the new origins of Boba Fett do not contradict previous material, because the previous material wasn't even technically true in the first place, as far as the films are concerned. EU has its own canon/non canon status, and, for example, Splinter of the Mind's Eye has been removed from it.
That is it for storyline continuity, though it all causes some debate. it is worht noting that published storyline is not the only source of canon- the above material just regulates the published stuff. Technically, what is canon in the SW storyline is set as soon as GL thinks of it. It is not apparent, of course, until he actually says it.
However, this only applies to GL. Worth noting that RM's comments about, for example, sabre colours, are good indications but are NOT canon. Material in the website is not technically canon either, but it would be surprising if it was wrong.