The article deals with a fantasy genre, the one of the main genres of the world literature. The author tries to explain this literary phenomenon by generalizing the definitions given in critical literature to the fantasy genre. The author also pays attention to the genre classification of a fantasy, and divides them into nine parts. In the article, post-apocalyptic motives which recently take the leading positions in fantastic literature are also mentioned. The history and a condition of post-apocalyptic motive were investigated, works by science fiction writers were analyzed.