"Steampunk" is Victorian science fiction. It references a time period in the industrialized 19th century. Historically , this is the period were many key developments took place leading to the modern world, such as mechanized manufacturing, extensive urbanization, telecommunication, office life and mass-transit. "Steampunk" uses these existing technologies to further advance the 19th century.
While the 19th century had its own science fiction famously embodied by the works of Jules Verne and H.G. Wells, the term "steampunk" was not introduced until the late 1980 by J.K. Jeter (author of sci-fi books such as Noir, Star Wars, Real Dangerous Girl, et al). Jeter used it humorously to describe a grouping of stories set in the Victorian period written during a time when near-future cyberpunk was the prevailing form of science fiction.
The basic rule in outfitting for "steampunk" is to start with Victorian style clothing and then add gears such as flywheels, pistons and goggles. The gear on its own is not especially “steampunk” but when put to use in 19th century machinery it become a key icon of the genre.
The genre possesses a life of its own that draws in fans from countless directions and backgrounds into a world where fashion is tailored to the individual, goods are made to last, and machinery is still regarded as a thing of visual majesty.
Information gathered from Steampunk 101