Hey there!

I should say first that this is sometimes a hotly contested topic, and there are no set rules. It's more important to write your story the way it needs to be written than to worry about word-counts. But that being said, I can offer some basic guidelines:

Flash-fiction is usually considered to be a piece under 1000 words.
A short story is usually considered a piece under 7500 words.
A novelette is 7500 to 17500
A novella is 17500 to 40000
And a novel is anything over 40000 words.

But in the end, it is really up to the author to decide what they want their story to be. I am sure there have been novels under 40000 words and novellas over. Just as I am sure there have been short stories over 7500. All of the limits can be adapted to fit the author's tastes and needs. Still, I think that is an acceptable guideline for word counts. Please, any one feel free to step in here and argue with me if they think I am wrong or have a different opinion. Healthy discussion is always a good thing.

Ali