Steam Punk (cyberpunk) adapts the style of the 19th century. It's a cross between Victorian era Britain  and science fiction or fantasy. The styles are based on Victorian  fashion, culture, and architectural style and art. Steampunk was the coined  phrase given to this style of genre, which originated in the late 
1980's by author K. W. Jeter.
There are two types of Steampunk: 
Historical Steampunk and Fantasy Steampunk.  
Historical Steampunk tends  to be more "science fictional": presenting and alternate history, 
presenting real locales and persons 
from history with different technology. 
Fantasy Steampunk, on the other hand, tends to present Steampunk in a complete
imaginary fantasy realm. Often populated by legendary creatures coexisting in
a Steam-era or anachronistic technologies. 
Although originally conceived as being Victorian-era science fiction only, the term has become common use 
for many related forms of speculative fiction set in the pre-electric age era. 
Steampunk was influenced by, and often  adopts the style of the 19th century scientific romances 
of 
Jules  Verne, H. G. Wells, Mark Twain and Mary Shelly.
One of the earliest mainstream manifestations of Steampunk was the original CBS television series
The Wild wild west.  (1965-1969).
Steampunk is usually depicted in the Victorian and Edwardian  eras, prior to a lot of electricity. Since the 1990's the Steampunk label  has expanded to include the fantasy world that rely heavily on steam or  spring powered technology.
The gnomes and goblin in world of Warcraft also have technological societies
that can be described as Steampunk.  
John Clute and John Grant have introduced the category "gaslight romance" Steampunk stories
are most commonly set in the romanticized, smoky , 19th century London.
Example: the early years of 
Dracula, Jekyll and Hyde, Jack the Ripper, Sherlock Holmes.  
Steamgoth is a dark side of Steampunk.
Another setting is Western Steampunk, which overlaps with both Weird West and Science fiction West. 
Several current utilitarian objects are used or modified to be used in 
Steampunk style. 
Polished brass, iron,  wood, and leather, computer keyboards, 
and electric guitars as long as  these items are consistent with the Victorian era.
Will Steampunk break into the  mainstream and have staying power? 
The Clockwork Century blogglist  believes it will..
Steampunk comes from a philosophy of salvage  and customization. 
Secondly Steampunk is participatory , and yet is  simultaneously rebels and its inconclusiveness
The BBC series Doctor Who also incorporates Steampunk elements, 
reference the Doctor's time machine.
Because  of the popularity of Steampunk with people in the 
Goth , Punk and Rivet  cultures, there is a growing 
movement toward establishing Steampunk as a  culture or alternate lifestyle. 
Steampunk fashion has no guide lines, but tends to synthesize modern styles 
influenced by the Victoria era. 
Examples: Corsets, petticoats, and bustle, suits and vests,
coats and spats, boots, large sole, buckles, straps, 
or military-inspired garments. 
Items often used are timepieces, parasols, 
flying/driving goggles, and ray guns. 
Several fashion styles have spun off of the Steampunk ideals...
Lolita fashion in Japan to the aristocrat
styles, and goth.
Steampunk music is less defined and tends to apply to any modern musicians 
whose music evokes a feeling of the
Victorian era or Steampunk. 
A slogan for Steampunk:
What the past would look like if the future had happened sooner.