From The Steampunk Wiki at Brass Goggles

What is steampunk?

That is a very hard question to answer and often argued. Steampunk is regarded to have started as a literature genre, based on tales written or set in the Victorian era that involved weird and fantastical uses of the technologies of the day beyond their recorded purpose. Good examples are Jules Verne's tales of Captain Nemo's Nautilus submarine, or H.G. Wells's "War of the Worlds" and "The Time Machine".

The concept has since expanded to a roleplaying and lifestyle trend, where steampunks will use aspects of steampunk in their lives and activities, whether it is their home, clothing, attitude or skillset.

The punk aspect too has developed from a "mad scientist" perspective, to alternative realities that change historical facts. Steampunks will often endeavour to act and use items in novel ways, combining things in new ways to develop or represent new inventions, be it a ray gun, a rocket pack or a telephonic image replicator.

There is no one definition of what is steampunk, but the following things are often present in anything regarded to be 'steampunk':

  • Victorian-era style
  • steam-powered technology
  • punk attitude, breaking away from the norm

What do you call those in the steampunk community?

A steampunk or steampunker, though Sir or Madam will often do nicely.