What is a Novella?
A novella is a stand-alone piece of fiction that is shorter than a full-length novel but longer than a short story. Novellas incorporate many narratives and structural elements of novel-length stories, but they often focus on a single point of view.
It would be reductive to think of the novella (or novelette) as simply a short novel, as there are many other hallmarks of the form that separate it from novels beyond just a basic word count.
If you're interested in writing a novella, there are a few things you should keep in mind.
First, novellas need to have a strong central plot. This is because readers will have less patience for subplots and side characters in a novella than they would in a novel.
Second, novellas should be focused on one or two main characters. This doesn't mean that you can't have supporting characters, but the focus should be on the protagonists.
Third, novellas need to have a tight structure. This is because there's less room for meandering in a novella than there is in a novel.
Finally, novellas need to have strong writing. This is because readers will be expecting high-quality prose in a novella since they're investing less time in reading it.