The Dark Mark is the symbol of Voldemort and the Death Eaters. It looks like a skull with a snake coming out of the mouth in place of the tongue. As a spell it is cast by a Death Eater whenever he or she has murdered someone. The spell used by Death Eaters to conjure the Mark is Morsmordre. It first appears in Goblet of Fire and is described as a "colossal skull, comprised of what looked like emerald stars, with a serpent protruding from its mouth like a tongue." Once in the sky it was "blazing in a haze of greenish smoke."
Dark Marks are also branded on the left forearm of the closest followers of Voldemort. The mark serves as a connection between Voldemort and each who bears it, he can summon them by touching his mark, causing it and those of his followers to burn and change colour. Death Eaters can summon Voldemort in the same fashion. Following Voldemort's ultimate defeat, the Dark Marks on his Death Eaters fade into a scar "similar" to Harry's.