"The Red-Headed League" is a detective story, part of a series of stories focusing on the cases of Sherlock Holmes, the great fictional detective. As a writer, Sir Arthur Conan Doyle is credited with popularizing the idea of the detective story (although Holmes was not actually the first fictional detective). A type of mystery story, the detective story is usually considered a subgenre in itself, the idea being that a consulting detective is presented with a case from a client, which he then must solve.
"The Red-Headed League" was first published in The Strand magazine and was intended to stand alone. While all the stories in the Sherlock Holmes canon are connected through their central characters, there are many inconsistencies, and it is not necessary to have read any of the others in order to understand the context of this one. The general premise is that Holmes's companion, Dr. Watson, the narrator of the story, is in fact writing the story up in retrospect for publication in The Strand: the story therefore has an interesting metatextual aspect. Watson narrates the story in the first person, in the past tense, and has a limited perspective—the story does not know anything that Dr. Watson could not know. As a character, Watson serves to illuminate the central character, Holmes himself, whose brilliance is demonstrated through his solving of the mystery.