As Patrick said above, according to the rules of Nebula Awards, the Science Fiction and Fantasy Writers of America (SFWA is the correct abbreviation, despite the two Fs) considers a novella to be 17,500 - 40,000 words.
40,000 and above is novel territory.
However, this designation is more flexible in the world of other genres or general fiction.
Also, since the popularization of NANoWriMo, many consider a novel to be 50,000 and above.