When I was explaining what Steampunk is to the vendors for my wedding, I found the most simplistic description was, "Victorian with attitude." However, this is not an accurate description of the entirety of the Steampunk subculture.
"A genre of science fiction that typically features steam-powered machinery rather than advanced technology." - Google Search
"A re-imagined history of a future that never was." - Something people say that raises more questions
"Really cool." - What people say when they don't know what to say
Steampunk started out as a literary genre, or at least a subculture of science fiction. Jules Verne, H.G. Wells, and Mary Shelley are all cited authors that gave the culture traction. This then influenced, in some order, music and fashion. Movies like "Wild Wild West" starring Will Smith and "League of Extraordinary Gentlemen" starring Sean Connery are both frequent examples of the aesthetics of Steampunk as used in media.
The term "Steampunk" was originally coined in a letter from K.W. Jeter to a science fiction magazine in order to describe Victorian fantasies. Flying airships, mechanical limbs, time pieces, clockworks, protective eyewear, and tesla coils became commonplace among the culture that was created.
Beyond its assumption of, "no electricity," Steampunk has no rules (thus, 'punk'). The universe in which characters exists can range from person to person based on how they imagine the technology would work. Airships could fly via magic, steam technology, or some law of nature that is built into that character's world.
This is what makes it difficult to explain Steampunk to those unfamiliar with the culture - it's personal. Once you grasp the most basic concepts of Steampunk, you make it your own. The next encouraged step is to get involved in the community (most easily searched for on Facebook or at the Steampunk Empire).