For quick and fun explanation, go watch this video: http://www.1stfloorflat.co.uk/2012/04/steampunk-video-freebie-blog-background.html
Whole books have been written about Steampunk, so I’m not going to even try to explain all the nuances and splinter groups.
I’m going to use Goth as a reference point. When you think of the word Goth, you might think of a lifestyle. You might think of certain colors, such as black and silver. You’ll probably think of certain objects, such as skulls and black roses, and when you think Goth fashion, you think of dark clothing using those items as accessories.
In a similar way, Steampunk isn’t a single thing. For a few it’s a lifestyle, a hobby or expression. For most it is just a certain look and style.
In the 1980s, it started as a Science Fiction genre that encompassed alternate realities where our current or future world still used steampower and technology dating from the time of Jules Verne. Hollywood envisions it as a reality where fantastic machines and technology were available during Victorian times –think of the movies The League of Extraordinary Gentlemenor Wild, Wild West or the latest in Sherlock Holmes movies.
Either way, apply that look and feel to crafts. You have vintage images centered around machines and Victoriana. You have layers and layers of gears and goggles and watch parts and clocks and the bits and pieces associated with early airplanes and old ships and automobiles. You can make anything Steampunk by adding these kinds of bits and pieces—i.e. the giraffe and flowers stamps used in my advertisement are Steampunk because they are made up of gears and sprockets. In some cases, it is very difficult to define-The Lady with a Mirror is Steampunk, but it is hard to say why. Something about the pose and attitude.
I think that is probably enough of an explanation to get you going. Here are a few links—a Steampunk craft sight, and some articles that explain the whole phenomena in more depth, if you are interested.
What Is Steampunk? by Garrett Mickley