We usually explain it to people as "Victorian Meets SyFy". As you will see in the quote below, we are right on!
"In three short words, steampunk is Victorian science fiction. Here "Victorian" is not meant to indicate a specific culture, but rather references a time period and aesthetic: the industrialized 19th century. Historically, this period saw the development of many key aspects of the modern world (mechanized manufacturing, extensive urbanization, telecommunications, office life and mass-transit), and steampunk uses this existing technology and structure to imagine an even more advanced 19th century, often complete with Victorian-inspired wonders like steam-powered aircraft and mechanical computers."
--G. D. Falksen
as printed in the Ohayocon 2011 program guide