truepenny's got a bit more on genre definitions.
To which I responded:
...my pocket definition of science fiction has nothing to do with science, or technology. I define it as "the literature of testing to destruction." Which neatly includes sociological science fiction--testing societies to destruction--and "if this goes on" stories as well as stories of advanced technology.
And you know, I still like that definition. For one thing, it firmly puts Margaret Atwood and Kurt Vonnegut in the us pile and Michael Crichton in the them pile, just the way we like it. And since it's all about sorting things into piles...
...oh, wait, no it's not.
I agree with truepenny in terms of the need for a vocabulary for critical discourse. I'm a descriptivist rather than a prescriptivist, and so I tend to look at things and go "Oh, that's X" and then have to come up with reasons to justify my snap judgement. The Cooler? Fantasy. Definitely. In a subtle underhanded American Magic Realism sort of way. Star Trek? Mostly not science fiction.
I tend to subscribe to papersky's definitional subsets: all fiction is fantasy, and it crunches down from there into smaller and smaller subsets--Science fiction is fantasy that plays by a number of generally accepted rules, and in which the fantastic tropes follow some clearly defined guidelines. (Alternate history is okay, time travel is okay, psionic powers are kind of iffy at best.) Mimetic fiction is science fiction that follows even tighter guidelines--the physical rules are exactly as they are on our earth, say, and everything is just like here, except maybe there's a big city on the east coast of the US somewhere between Washington and Boston in which there's a certain precinct with a red-haired cop in it....
Or a hotel in New Hampshire inhabited by a person in a bear suit. Whatever. You know what I mean.
So yeah, I can fliply say that there's no difference between SF and Fantasy, and what I mean by that is that there's no difference between red and orange. Or orange and yellow. But there is clearly a difference between yellow and red, right? So somewhere in the middle of orange, there must be a line you can draw and say "Everything to the right of this is red, and everything to the left is yellow."
Except you can't.
You can draw all the color charts you want, and you can establish what a 50% mix of red and yellow look like at any number of saturations, and it still doesn't matter. There will always be some things we can clearly identify as red (Robert Forward's Dragon's Egg, say, Christ imagery and all, doesn't have a hell of a lot of yellow in it) or yellow (Ummm...*pulls Mercedes Lackey out of a hat and dusts her off*) and yeah, there you go, there are clearly two different things.
The thing is, in real life, unlike in the human brain, categories aren't so tidy. The bleb and interfere, and nobody's going to look at that 50/50 mix and say "it's red!" unless they're pushing an agenda. It's orange, and they know it's orange.
I don't actually have a dog in this fight. I like science fiction and fantasy. I write both. I try to write both as literature (by which I mean, I take my writing seriously: I may in fact be a hack, but if I am, then I'm a hack who tries real goddamned hard to write the best books she can), and also as cracking good stories, because I do not think the two things are mutually exclusive. I see some cosmetic differences between the two, but I don't have a philosophy going of what SF or Fantasy ought to be, and left to my own devices, I generally refer to "SFF" or "speculative fiction."
It is indeed commercial fiction, and nihilistic_kid's comment that most of it is crap is taken under advisement. And frankly, I haven't got a thing to counter that with except Sturgeon's law and the fact that, like Ursula Le Guin, I love my ghetto. And I love its breadth and depth and quirkiness, and all the weird little corners it has, and I have absolutely no investment in saying science fiction should be this or should be that. I haven't got an ideology of speculative fiction; I'm happy to cast my net over the New Weird and the New Pulp and just about all the rest of it. And I love the ways it edges out into the mainstream unnoticed and then doesn't get called "science fiction" in much the same way that Joanna Russ talks about the suppression of women's writing, because if it's any good then it's not science fiction.
And yeah, you know, some of it is imperialist apologism, and so is some fantasy, and I'm interested in undermining those tropes and trying to say some interesting things about our cultural assumptions too. And I find fantasy and sf very useful for that, because they are so very flexible, and sometimes it's easier to talk about something if you can divorce yourself from reality a little.
I dunno. I don't really have a thesis statement here, or anything to prove. But man, I think--science fiction writer, fantasy writer, fantasist, whatever you want to call him--Theodore Sturgeon could write. And if you say well, there was only one of him, I refer you to Ms. Russ's book. And Ursula Le Guin. Who can also, dare I say it, write.
Send in the talking bunnies, man.