The theory behind steampunk is that most of the inventions of the modern world could have been created using the technology available during the Victorian Era. They would have been larger, of course, and less efficient; creating the Internet using Babbage engines and the telegraph would be one such example.
Lots of impossible yet usually plausible devices are prominent in steampunk. This follows directly in the traditions of HG Wells, Jules Verne, and other classic authors who gave us things like steam-powered time machines.
As a result, an entire sub-genre of alternative history fiction, mixed with sci-fi, comes into play. The name "steampunk" is directly related to the term "cyberpunk"; in both, technology is greatly accelerated, and the huge gaps between the "upper class" and the "lower class" are made evident. Of course, the lower class begins to question this, rebels a bit, and the rest should have been history.