Straight from the mouth of wikipedia (and you know how reliable they are);
Alternate history or alternative history is a genre of fiction consisting of stories that are set in worlds in which history has diverged from the actual history of the world. It can be variously seen as a sub-genre of literary fiction, science fiction, and historical fiction; different alternate history works may use tropes from any or all of these genres. It is sometimes abbreviated AH. Another occasionally used term for the genre is “allohistory” (literally “other history”).
My writing is usually focused on my interests. I remember having one especially gifted Western Civ professor at Longwood College (now Longwood University) who introduced me to those madmen of the Classical Age: the Greeks and the Romans. They were a fairly busy people: building theaters, establishing myths and waging war for hundreds of years at a pop. When I got around to my English Literature courses (taught by another wonderful professor at Longwood), she encouraged me and my fellow students to embrace the idea that mythology was not necessary an “ancient” idea. She just loved Battlestar Galactica (sorry, the original really old TV series) and the idea of ancient heroes transplanted in a future setting. She called it “crossing genres”.
So I really enjoy the idea of using history as a template for storytelling. I’m not the only one here. Lately, there will be a slew of movies using famous historical figures in unusual settings. Try Pride and Prejudice and Zombies, for starters. How about Abraham Lincoln, Vampire Hunter? The Raven stars John Cusack as Edgar Allen Poe challenged to solve a series of murders based on his own works (I recognized The Tell Tale Heart and The Pit and the Pendulum as some of the murder scenes in the previews). More recently I read Ganymeade, which takes place post-Civil War and suggests a steampunkish America where a submarine is used to battle Texas airships and gunboats.
Alternate history comes in many different forms. Sometimes you don’t even realize you’re reading it. For example, there’s science fiction (War of the Worlds and 20,000 Leagues Under the Sea suggest modern battles against aliens and a naturalist-turned-terrorist).
My works fall into the alt history category. It’s a hard sell to agents and publishers. I just like the idea of taking something that happened and changing it up. Then it’s not the same old boring history. It becomes something different and exciting.