A novelette is considered by most publishers to be a piece of mid-length fiction between 7500 and 17,500 words in length. Any story under that size is considered a short story, and anything longer a novella or novel.
The novelette and novella are considered by many to be the "natural" length for many science fiction stories due to SF authors needing a bit more space to adequately create a good settings for the reader and (in the case of hard SF ) to incorporate technical detail about how the new technologies etc. in said futuristic/alien world works.
Unfortunately, many magazines, webzines, and anthologies don't publish work in the novelette range, and those that do may be reluctant to accept such work from unknown authors because a single novelette may cost a big chunk of their fiction budget for an issue and not attract many new readers to compensate. So, beginning writers will find their mid-length work harder to place than their short work.
