In the course of my reading, reading about writing, and occasionally even reading about reading, I tend to group information into at least three, rather than two, distinct categories. For me, canon is what is actually written down in the book, there forever, immutable. It describes the universe of the story, a rigid framework from which all other things hang. Unless the author engages in some serious retconning (and they shouldn’t!), canon will be there forever.
Semi-canon is a more complex beast. Included in here are things hinted at but never spoken in any book, or things the author has said but never included into anything. Subject to change, they might be regarded by some as “Word of God”, but for me they’re more strongly worded suggestions. That’s what you meant? That’s nice, but that isn’t what I read. That’s how you pictured that? That’s nice, but I’ve always seen it this way. And that’s how I want my random pronouncements to be viewed – particularly since anything not written down is, for me, potentially subject to change. Semi-canon comes on a great sliding scale from the practically canon solidity of “yeah, this is the timeline I’ve been using since Day One to keep everything coherent” down to the ephemeral “hey, wouldn’t it be cool if the reason for X was Y? I just thought of that!”
Fanon, meanwhile, is what one or more fans have started to take as read. It has a lot of value all its own – but no fanfic writer should feel bound by its constraints. There’s a whole world out there!
And of course, there’s always the Great Disaster, the fatal moment when a beloved series jumps the shark. As far as my own personal canon goes, anything that happens after that is semi-canon at best. And I hope that if anything I write goes that way, any readers will be sensible enough to take the same view. (And possibly also to warn me that I’ve gone insane. Insane!)
Addition 2013-05-08: Forgot to add, but share your own thoughts! This is just my view!