It is crucial when submitting your manuscript to a publisher, or when promoting your book online, that you understand the conventions of book lenths, and that you are able to correctly label your book.
Nothing screams 'amateur' more than a writer who calls their novella a novel, or vice versa - and you certainly don't want to annoy readers by promising them a novel, but only delivering a novelette.
So what are the publishing industry conventions?
Micro Fiction: up to 100 words. This takes a lot of discipline, and a masterful way with words. For examples, check out www.100wordstory.org.
Flash Fiction: between 100 and 1000 words. For more information on how to write flash fiction, check out this article on Writer's Digest.
Short Stories: between 1,000 and 7,500 words. These are the stories you usually find in magazines - sadly a decreasing market, but still a lucrative form of income for those who manage to crack it.
Novelettes: between 7,500 and 20,000 words. The less pernickety do away with this term and simply lump stories of this length in with...
Novellas: between 20,000 and 50,000 words. These lengths are currently very popular with readers who read on electronic devices or have shorter attention spans.
Novels: Anything between 50,000 and 110,000 words. This is the most common length for print books. Anything less is too short to warrant printing costs, and anything too long becomes too expensive to print (unless you're JK Rowling and are guaranteed to reach bestseller status).
Added bonus: Books between 45,000 and 65,000 words are often referred to as 'category length' after Mills & Boons' category novels. This is a popular length for romances, traditional paperback westerns, and other shorter genre fiction.
Epics & Sequels: anything over 110,000 words. If you're a first time writer, the likelihood of any publisher offering to publish a novel of this length is slim to none (see the cost of printing above). And in this post-MTV generation, there aren't that many readers with the kind of attention spans required to take on any book this long!