I've read some places that Trek canon is open to debate. I've read other places that the television series and movies are canon while novels, comics and games are not. It seems to me it should be either one or the other. So what is Trek canon? Is it simply individual opinion or something that is easily defined? And how is/was it determined?
Trek canon is the live action shows and movies. It's supposed to be the body of work that new Trek keeps consistant with. In cases of rewrites or retcons, new canon overwrites the old.
The novels aren't canon, but they have their own ongoing continuity around and beyond the episodes and movies. Since there isn't going to be any new TV/film Trek in the TOS/TNG/DS9/VOY continuity, they're not going to be contradicted in the way that some older books were.
There are a few episodes that have been mostly ignored or forgotten, like TOS: "The Alternative Factor" or VOY: "Threshold" - although there is no official list of what episodes are canon and what aren't, or which bits count and which don't. CBS don't take it as seriously as the Trek fans do. It's more or a loose mythology.