Some helpful instruction from SFWA, in the context of word counts and story types:

1. Short Story: less than 7,500 words;
2. Novelette: at least 7,500 words but less than 17,500 words;
3. Novella: at least 17,500 words but less than 40,000 words
4. Novel: 40,000 words or more.

From the new Nebula rules (which I learned about from Jay Lake).