People still debate the characteristics something should include to be classed as steampunk, but there are certain elements which people are pretty much agreed on.
Steampunk is a sub-genre of science fiction, alternate history, and speculative fiction that came into prominence during the 1980s and early 1990s. It specifically involves an era or world where steam power is still widely used—usually the 19th century and often Victorian era Britain (or Victorian-esque as in my Pax Britannia books —that incorporates prominent elements of either science fiction or fantasy.
Works of steampunk often feature anachronistic technology or futuristic innovations as Victorians may have envisioned them; in other words, based on a Victorian perspective on fashion, culture, architectural style, art, etc. This technology may include such fictional machines as those found in the works of H. G. Wells and Jules Verne or real technologies like the computer but developed earlier in an alternate history.
Other examples of steampunk contain alternate history-style presentations of 'the path not taken' for such technology as dirigibles, analogue computers, or such digital mechanical computers as Charles Babbage and Ada Lovelace's Analytical engine (and the Babbage Engines and Turing Machines of my own books).
Various modern utilitarian objects have been modded by individual artisans into a pseudo-Victorian mechanical steampunk style, and a number of visual and musical artists have been described as steampunk.
If you'd like to learn more about the steampunk movement, then you could do a lot worse than to order a copy of S. J. Chambers' and Jeff VanderMeer's The Steampunk Bible: An Illustrated Guide to the World of Imaginary Airships, Corsets and Goggles, Mad Scientists, and Strange Literature.