A novella is a short novel. They are usually divided from novels because of their word count (for example, literary contests that include separate categories often divide them that way). The problem is that, to my knowledge, there is no accepted standard for the difference. For my own use, I divide them in this way:
- I prefer to try for a minimum of 20k words (k = 1,000)
- If a work passes 40k, then it is more likely to be counted as a novel than as a novella.
In my opinion, novellas always seem to have a different feel from novels. They tend to feel more like an extended short story. Sometimes, though not always, there is more of a focus on details of plot over character development, or vice versa. Also, because of the shorter length, novellas are typically meant to be read in just one or two sittings.
Here are two examples of prominent novellas: