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. work in said futuristic/alien world.
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.