Best Novel is pretty self-explanatory. Novels are defined as works over 40,000 words. Short Story is for works under 7,500 words. Novelette covers stories from 7,500 to 17,500 words. And Novella is for stories longer than 17,500 words, but shorter than 40,000.
Novellas are so long they tend to fill up most of an average issue of a magazine, but too short to be published alone as a book, so typically only a few make it into print each year, compared to hundreds of short stories and novelettes.
Last year I had the honor of editing three extraordinary novellas in the Wild Cards books. Unlike the magazines, however, Wild Cards doesn't include a contents page where the stories are broken down and labelled as novellas, novelettes, and short stories, so even our Wild Cards fans may not be aware that these WERE novellas... especially since two of them were published as interstitial narratives, and the third was broken into four parts.
"Jonathan Hive," by Daniel Abraham, from INSIDE STRAIGHT,
"Political Science," by Walton (Bud) Simons & Ian Tregillis, from BUSTED FLUSH,
"Double Helix," by Melinda M. Snodgrass, from BUSTED FLUSH.
INSIDE STRAIGHT was published in January of last year and BUSTED FLUSH in December, so all these stories are eligible for this year's Hugos. All three of them were terrific, in very different ways. I urge you to remember them when filling out your nominating ballot:
Thanks for your consideration.