It's...tricky to define. I tend to think of cyberpunk as the intersection of dystopia and transhumanism. It tends to feature captalist dystopias, full of megacorps and consumerism - if there's a government, it's a figurehead only, as ultimately, the bottom line is all that matters. Large class divides, an emphasis on using technology to augment human ability, and a focus on computers/the Net. It's also a lot about style - black leather, chrome skyscrapers, mirrored sunglasses. Popular movie examples are Blade Runner, Hackers, and the Matrix. Anime would be Ghost in the Shell (all the versions), Serial Experiments Lain, Bubblegum Crisis.

This is pretty much my favorite genre /ever/, if you couldn't tell >.>