Steampunk is a subgenre of speculative fiction, usually set in an anachronistic Victorian or quasi-Victorian alternate history setting. It could be described by the slogan "What the past would look like if the future had happened sooner." It includes fiction with science fiction, fantasy or horror themes.
Medieval Steampunk: Speculative fiction set during the Middle Ages.
Victorian Steampunk: A modern Science Fiction work (post-1930s) that is set in the early parts of the industrial revolution.
Western Steampunk: Science fiction set in the American Old West.
Industrial/Modern Steampunk: Science fiction taking place in the late industrial age, early modern age; i.e. World War 1, World War 2