To quote directly from The 2009 Guide to Literary Agents: "speculative fiction that changes the accepted account of actual historical events, often featuring a profound "what if?" premise"
This subgenre crosses between speculative fiction and historical fiction. It uses actual history and forces it down a road it didn't actually go, whether introducing alternate technologies, different outcomes, choices made or not made, or introducing fictional characters into the mix. Alternate history, because it crosses two major genres, has the potential for a wide fan base.
Examples and Resources:
Alternate History Directory has a large list of fiction, essays, short stories, and such.
Uchronia: The Alternate History List is another great resource.
Flashlight Worthy posts a book list of The Best Earths That Never Were
io9 shares a brief history of alternate history fiction.
Goodreads has a listing of books classified as alternate history.
Alternate history shouldn't be confused with time travel stories, though they are very close. Time travel stories are a separate sister subgenre which I'll cover later on.