So what is Steampunk? The simplest explanation I can come up with is Victorian Futurism: it is a mixture of late 19th century / early 20th century fashion, industrial revolution (steam power) technologies and Sc Fi. As in many things, it began as a literary genre, in this case in the 60's, highly influenced by Jules Vernes and H.G. Wells.
What does this mean for costumers? Top hats, goggles, pocket watches, brass buttons, spats, vests, jackets and for girls, short bustled skirts, corsets and bloomers.
The one thing that can make any period costume Steampunk is the addition of clock gear (which you can easily find on eBay).
Need an example? My favourite Steampunk outfit to date is Alisa's Pirate Steampunk outfit.
Gold Steampunk Ironman Inspired Pendant by Matthew Nix
Matthew Nix from The Watchman's Shop over on Etsy is one of them. He makes beautiful Steampunk pieces (rings, pendants, money clips) from metal and fused glass, and he never makes the same piece twice!
If Steampunk is something that might interest you (costume wise), I suggest you watch the Threadbanger Steampunk Week videos. There are many interesting ideas, and their projects are quite easy to make!
And if you are really into Steampunk, head over to Steampunk Magazine for more information, stories and patterns on what has become a real Artistic Movement.
If you believe in the Many-worlds Theory, who knows, perhaps there truly exists a Steampunk world.