Adventure Fiction
Action-adventure fiction appeals mainly to male readers. It features physical action and violence, often around a quest or military-style mission set in exotic or forbidding locales such as jungles, deserts, or mountains. The conflict typically involves commandos, mercenaries, terrorists, smugglers, pirates, and the like. Stories include elements of courage, male bonding, and betrayal, as well as lore on technology, weapons, and other hardware. A thriller is a story intended to evoke strong feelings of suspense and danger, usually involving a high-stakes hunt, chase, or a race against time. Thrillers often involve espionage, crime, medicine, or technology. -- Review from http://en.wikipedia.org/wiki

Detective & Mystery Fiction
Detective fiction has become almost synonymous with mystery. These stories relate the solving of a crime, usually one or more murders, by a protagonist who may or may not be a professional investigator. This large, popular genre has many subgenres, reflecting differences in tone, character, and setting. Mystery fiction, technically involving stories in which characters try to not discover a vital piece of information which is kept hidden till the climax, is now considered by many people almost a synonym for detective fiction. The standard novel stocked in the mystery section of bookstores is a whodunit. Crime fiction stories, centered on criminal enterprise, are told from the point of view of the perpetrators. They range in tone from lighthearted "caper" stories to darker plots involving organized crime or incarcerated convicts.

Fantasy Fiction
Fantasy fiction features stories set in fanciful, invented worlds or in a legendary, mythic past. The stories themselves are often epics or quests, frequently involving magic. The enormous popularity of J.R.R. Tolkien's Lord of the Rings trilogy and J.K. Rowling's Harry Potter novels demonstrates the wide appeal of this genre.

Historical Fiction
To be deemed historical, a novel must have been written at least fifty years after the events described, or have been written by someone who was not alive at the time of those events (who therefore approaches them only by research).

Horror Fiction
Horror fiction aims to evoke some combination of fear, fascination, and revulsion in its readers. This genre, like others, continues to evolve, recently moving away from stories with a religious or supernatural basis to ones making use of medical or psychological ideas.

Children's Literature
Generally defined as ages 5-11 or grades K-6.

Nonfiction
Non-fiction is an account or representation of a subject which is presented as fact. This presentation may be accurate or not; that is, it can give either a true or a false account of the subject in question. Examples include: essays, journals, documentaries, histories, scientific papers, photographs, biographies, textbooks, blueprints, technical documentation, user manuals, diagrams and some journalism.

Romance Fiction
Romance is currently the largest and best-selling fiction genre in North America. It has produced a wide array of subgenres, the majority of which feature the mutual attraction and love of a man and a woman as the main plot, and have a happy ending.

Science Fiction
Science fiction is defined more by setting than by other story elements. With a few exceptions, stories off of Earth or in the future qualify as science fiction. Within these settings, the conventions of almost any other genre may be used. A sub-genre of science fiction is alternate history where, for some specific reason, the history of the novel deviates from the history of our world. Pavane (1968) by Keith Roberts was an influential early alternate history, Harry Turtledove's The Guns of the South a popular example. Of late, alternate history has come in its own as distinct and having an independent existence from science fiction generally.

Teen / YA Literature
Reading level is generally from ages 12 -20, or grades 6/7 and up. Of course, teen lit is a great genre for adults!

Western Fiction
Western fiction is defined primarily by being set in the American West in the second half of the 19th century, and secondarily by featuring heroes who are rugged, individualistic horsemen (cowboys). Other genres, such as romance, have subgenres that make use of the Western setting.

Note: Definitions cited from http://en.wikipedia.org