A mix of film noir and science fiction, generally used to decribe media. Cyber Punk is usually about hackers, or some form of sentient computer programming (like and AI). The stories are usually told with a very urban noir style.
As far as a look, it's hard to put a figner on it. I dont think any one look can be described as cyber punk, its more of an attitude. Black jeans, tight t shirt with holograms, and leather jackets are a good start. Maybe toss in some Chuck Taylors, or combat boots. Some goggles or head phones might make good accesories.
Neuromancer is the greatest cyberpunk sotry ever told.