What's the criteria for a short story, novelette, novel, etc?

Where's the cut-off for these different classes(?) of stories? Like how long does a short story have to be before it becomes more than a short story? It seems like I heard somewhere that a novel has to be at least 112 pages (although that would be a pretty short novel) but I might be way off on that. Writers, what are your opinions? And are there any set rules that sort of govern this already?