Novel versus novella

I once heard a non-writer ask why an author would write a novella rather than a novel. My short, non-sexy answer is because the author has a contract for a novella rather than a novel.

My novellas--I'm working on my third now--have all been contracted at 25,000 to 30,000 words. That's roughly a quarter of one of my novels. (My novel contracts have been for 95,000 words--give or take, you understand.) So in writing a novella, I have to limit things: limit the time over which the story takes place, limit the number of characters, and limit the focus to pretty much the hero and heroine.