This is a question I get at lot: What's the difference between a novel, a novella, and a short story?
Well, according to the RWA's contest-category page:
A “Novella” is a work of fictional narrative prose in the romance genre of 20,000–40,000 words as determined by computer word count. Novella entries with word counts less than 20,000 or more than 40,000 will be disqualified.
A “Novel” is a work of fictional narrative prose in the romance genre of at least 40,000 words as determined by computer word count. Novel entries with word counts less than 40,000 will be disqualified.
And a short story would (generally) be anything under the 20,000 word count for a novella.
There are upper limits, too, typically where the next type of manuscript starts, but in general, publishers don't want to publish a book that's over 120,000 words. They will for a few reasons, namely if you have a very popular series (or are a very popular author, like Stephen King) and the publisher knows people will buy it. For instance, the longest Harry Potter book was the fifth one, the Order of the Phoenix, and it topped out at about 257,000 words. However, the final four in the series were all well over 150,000 words. The Deathly Hallows was also right around 200,000 words. If you're not J.K. Rowling, a publisher will either reject for being too lengthy, or see if you can split the manuscript into two or more, because, hey, why sell one really long one when you're not sure if readers will buy it, when you can chop it into two and have two shorter-but-successful ones.
The problem--even though it's not really a problem, per say--is that the word counts can vary per publisher. What may be classified as one type of manuscript in one place might be a different one somewhere else, or not accepted at all. For example, a publisher may make the starting word count for a novel 50,000 words. That is, anything under 50,000 words that's submitted would either 1) not be accepted or 2) classified as something different, i.e. a novella, if the publisher publishes them.
When I started out writing The Curse, I thought it was going to be somewhere in the 30,000 word range, not making it into "novel" territory according to the RWA standards, but more of a long novella. As of right now, it's not finished and already over 32,000 words. Will I make it to the magic 40,000 word count? I have no idea. If I do, great, if not, that's okay, too. I'd like to get it up there if possible, though, because I can reach a larger market that way. Plus, publishers charge more for e-books that are novel-length.
I hope that clarifies some of these common publication terms that are thrown around.