A book genre is a particular class or type of book, separated based on certain criteria, like the tone, content, or setting. There are many different genres and sub-genres, and the line between one and another isn't always very clear. Books may fit the criteria for several different classes, in which case they're called cross-genre works. There is also a field of study focused on how and why literary works are categorized as they are.
Common Book Genres
Literary genres are generally broken down into fiction and non-fiction. Common types of fiction include comedy, drama, history, romance, religious, fantasy, humor, mystery, science fiction; and common types of non-fiction include biography and autobiography, narrative, and speech. There are many sub-classifications, including things like satire and comedy of manners within comedy; historical romance and family sagas within history; and hard and soft science fiction. Some books are cross-genre, meaning that they contain essential elements of several distinct categories, like space westerns, which are generally set in outer space but contain elements of classic American Westerns, like cowboys.
Books are typically classified based on their tone and the focus of their content. For instance, a work that has a light, comic tone and is focused on a female protagonist finding a boyfriend would likely be a romantic comedy, while one that was more serious and chronicled the lives of members of a family during World War II might be a family saga or war fiction, and a collection of letters written by a historical figure throughout his or her life would be an epistolary autobiography. Some book genres also have specific types of characters associated with them, like knights in medieval fiction or cynical detectives in hard-boiled mysteries. The setting of a work can play a role as well if it's distinctive, like 17th century England or the American West in the mid-1800s.
Literary scholars often study book genres to try to determine why literary works are categorized in this way to begin with and how this division is carried out — for instance, the minimum criteria that must be fulfilled to definitively say that a work falls in one genre instead of another. They also debate which classifications are pure and which are mixed and how different ones relate to each other. Some take a more socially focused approach and look at whether genres reflect existing social hierarchies and the effects on class, race, or gender relations of categorizing books in a specific way.