A genre is a specific type of music, film, or writing. Your favorite literary genre might be science fiction, and your favorite film genre might be horror flicks about cheerleaders. Go figure.
In music, genre refers to musical style such as jazz, salsa or rock. In film or literature, the genre is determined by the subject, setting or plot of the story. When you are wandering around a book store, books are usually arranged by genre. Many genres, such as "mystery" or "fantasy," overlap in film and literature, while others, such as "romantic comedy" in film or "historical fiction" in books, are specific to one or the other.