A skull and crossbones is a symbol consisting of a human skull and two bones crossed together under the skull. Today, it is generally used as a warning of danger (usually in regard to poisonous substances).
The symbol, or some variation thereof, was also featured on the Jolly Roger, one of the many flag designs of European and American pirates; it is thought of as the stereotypical pirate flag, although it has historically been used for other purposes as well.
Traditionally, the crossbones behind the skull indicates poison, while the crossbones under the skull (the "Jolly Roger") indicates pirates or piracy.

Modern uses

Hazard symbol
Jolly Roger
Mr. Yuk
Sedlec Ossuary — a church made of skeletons that uses the Skull and crossbones symbol at some places
Totenkopf — "Dead (person)'s Head" insignia of former German elite military units.
Skull and Bones
Danse Macabre