Steampunk is a sub-genre of science fiction that includes aspects of steam-powered machinery, typical of the industrial features of the 19th century. Therefore, the Victorian era in England and the corresponding industrial development and western expansion in America are common historical settings. However, steampunk tends to be more science fiction than historical in genre. For example, the movie Wild Wild West (1999) is set in the West in the 19th century but the technology, still based around steam, is certainly more advanced than it would have been during that real historical time period. Therefore, this is a steampunk film that presents a science fiction or alternate history of the period. Another way to describe a work (in film or literature) like this is to call it "retro-futuristic." Steampunk literature, film, and art is based around the steam-powered industry of the 19th century but such works and artwork tend to include science fiction aspects.
Works written in the 19th century also can be categorized as steampunk. Jules Verne and H. G. Wells are two of the most common names mentioned in association with 19th century steampunk because their work was rooted in the science and industrialization of the time period but they also wrote in the science fiction genre. Clearly, Wells' time machine, although based on the technology of the time period, exceeded the actual technical knowledge of the time.
Another more contemporary example of steampunk is Philip Pullman's series, His Dark Materials. In The Golden Compass, the setting seems Victorian, but it is an alternative Earth. Electricity and steamships are mentioned but most of the other technology characteristic of the Victorian period is missing. The magic and spiritual aspects of the novel seem to supplement this lack and/or place it in a more indefinable historical setting. Such is the nature of science fiction. The work is considered steampunk because it has aspects of steam and the era (Victorian) during which steam-powered industry was prominent.