It is a skull and two femur like bones arranged in an x underneath or below the skull. Pirates (they guys in boats, not the WaR3Z D00Dz) commonly used it, typically on their flags where it was referred to as the Jolly Roger.
As pirates were seen as evil villainous characters, their symbols, including the skull and crossbones, have become synonymous with evil and nefarious activities. With the advent of modern teenage rebellion chic, they have maintained a presence in all sorts of forms in pop culture.
I just got a keen magnet of a skull and crossbones yesterday. I think its cool, but I think pirate stuff is cool in general.