Alternate History: speculative fiction that changes the accepted account of actual historical events, often featuring a profound "what if?" premise.
Arthurian Fantasy: reworkings of the legend of King Arthur and the Knights of the round Table.
Bangsian Fantasy: stories speculating on the afterlives of famous people.
Biopunk: a blend of film noir, Japanese anime and post-modern elements used to describe an underground, nihilistic biotech society.
Children's Fantasy: a kinder, gentler style of fantasy aimed at very young readers.
Comic: fantasy or science fiction that spoofs the conventions of the genre, or the conventions of society.
Cyberpunk: stories featuring tough outsiders in a high-tech near future where computers have produced major changes in society. It typically has countercultural antiheroes who find themselves trapped in a dehumanized future.
Dark Fantasy: tales that focus on the nightmarish underbelly of magic, venturing into the violence of horror novels.
Dystopian: stories that portray a bleak future world. Stories where the apocalypse occurs, whether in the form of a nuclear bomb, asteroids, disease, or even a political regime, fit this genre.
Erotic: SF or fantasy tales that focus on sexuality.
Game-Related Fantasy: tales with plots and characters similar to high fantasy, but based on a specific role-playing game like Dungeons and Dragons.
Hard Science Fiction: tales in which real present-day science is logically extrapolated to the future.
Heroic Fantasy: stories of war and its heroes, the fantasy equivalent of military science fiction.
High/Epic Fantasy: tales with an emphasis on the fate of an entire race or nation, often featuring a young "nobody" hero battling an ultimate evil.
Historical: speculative fiction taking place in a recognizable historical period.
Mundane SF: a movement that spurns fanciful conceits like warp drives, wormholes and faster-than-light travel for stories based on scientific knowledge as it actually exists.
Military SF: war stories that extrapolate existing military technology and tactics into the future.
Mystery SF: a cross-genre blend that can be either an SF tale with a central mystery or a classic whodunit with SF elements.
Mythic Fiction: stories inspired, or modeled on, classic myths, legends and fairy tales.
New Age: a category of speculative fiction that deals with occult subjects such as astrology, psychic phenomena, spiritual healing, UFOs and mysticism.
Post-Apocalyptic: stories of life on Earth after an apocalypse, focusing on the struggle to survive.
Romance: speculative fiction in which romance plays a key part.
Religious: centering on theological ideas, and heroes who are ruled by their religious beliefs.
Science Fantasy: a blend in which fantasy is supported by scientifc or pseudo-scientific explanations.
Social SF: tales that focus on how characters react to their environments--including social satire.
Soft SF: tales based on the more subjective, "softer" science; psychology, sociology, anthropology, etc.
Space Opera: a traditional good guys/bad guys faceoff with lots of action and larger-than-life characters.
Spy-Fi: tales of espionage with SF elements, especially the use of high-tech gadgetry.
Steampunk: a specific type of alternate history in which characters in Victorian England have access to 20th century technology.
Superheroes: stories featuring characters endowed with superhuman strengths or abilities.
Sword and Sorcery: a classic genre often set in the medieval period, and more concerned with immediate physical threats than high or heroic fantasy.
Thriller SF: an SF story that takes on the classic world-at-risk, cliffhanger elements of a thriller.
Time-Travel: stories based ont he concept of moving forward or backward in time, often delving into the existence of parallel worlds.
Urban Fantasy: a fantasy tale in which magical powers and characters appear in an otherwise normal modern context, similar to Latin American magical realism.
Vampire: variations on the classic vampire legend, recently taking on many sexual and romantic variations.
Wuxia: fantasy tales set within the martial arts traditions and philosophies of China.
Young Adult: speculative fiction aimed at a teenage audience, often featuring a hero the same age or slightly older than the reader.
And because some people crossover into the Horror/Paranormal genre:
Child in Peril: involving the abduction and/or persecution of a child.
Comic Horror: horror stories that either spoof horror conventions or mix the gore with dark humor.
Creepy Kids: horror tale in which children--often under the influence of dark forces--begin to turn against the adults.
Dark Fantasy: a horror story with supernatural and fantasy elements.
Dark Mystery/Noir: inspired by hardboiled detective tales, set in an urban underworld of crime and moral ambiguity.
Erotic Vampire: a horror tale making the newly trendy link between sexuality and vampires, but with more emphasis on graphic description and violence.
Fabulist: derived from "fable," an ancient tradition in which objects, animals or forces of nature are anthropomorphized in order to deliver a moral lesson.
Gothic: a traditional form depicting the encroachment of the Middle Ages upon the 18th century Enlightenment, filled with images of decay and ruin, and episodes of imprisonment and persecution.
Hauntings: a classic form centering on possession by ghosts, demons or poltergeists, particularly of some sort of structure.
Historical: horror tales set in a specific and recognizable period of history.
Magical Realism: a genre inspired by Latin-American authors, in which extraordinary forces or creatures pop into otherwise normal, real-life settings.
Psychological: a story based on the disturbed human psyche, often exploring insane, altered realities and featuring a human monster with horrific, but not supernatural, aspects.
Quiet Horror: subtly written horror that uses atmosphere and mood, rather than graphic description, to create fear and suspense.
Religious: horror that makes use of religious icons and mythology, especially the angels and demons derived from Dante's Inferno and Milton's Paradise Lost.
Science-Fiction Horror: SF with a darker, more violent twist, often revolving around alien invasions, mad scientists, or experiments gone wrong.
Splatter/Splatterpunk: an extreme style of horror that cuts right to the gore. This subgenre, which first appeared in the '80s, lives up to its name--explicit, gruesome violence.
Technology: stories featuring technology that has run amok, venturing increasingly into the expanding domain of computers, cyberspace, and genetic engineering.
Weird Tales: inspired by the magazine of the same name, a more traditional form featuring strange and uncanny events (Twilight Zone).
Young Adult: horror aimes at a teen market, often with heroes the same age, or slightly older than, the reader.
Zombie: tales featuring dead people who return to commit mayhem on the living.
As you can see, the choices are staggering. What that list can show you better than I can put into words is how hard it can be to nail down your subgenre. There are just so many possibilities. And this list only covers the fantasy/science fiction side of things.
Obviously it's much easier to just say fantasy or science fiction in your query letter and leave it at that. But so many stories are a little bit of this and a little bit of that when it comes to genre. A little romance, a little scariness, a little action thrills. A little contemporary mixed with a little of the weird. And suddenly your head is spinning. What genre do I pick?
So a couple of points: First off, other writers can give you pointers. I've learned this writing community is a wonderful place. Trust in friends and friends will help you out every time. Describe your stories to other writers and they can at least get you a consensus.
Next, YA, Adult, MG, and PB are not genres. They are age categories. When someone wants to know your genre for a contest or a query letter, saying YA doesn't answer the question. It's YA paired with Thriller or Horror or Historical Fiction, or whatever your genre may be. MG Adventure, Adult Science Fiction, YA Mystery are examples of the full and complete answers you need to give.
And last, it's not the end of the world if you get the genre wrong. Do the best you can to define the genre of your story, but the agent is the expert. When you sign with them, they can guide you on where your story fits.
Here's a more complete list of subgenres from Writer's Digest. And another list to muddy the waters from Cuebon.