What is the "Dark Mark"

The Dark Mark is a symbol used by Lord Voldemort and his 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. 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. In the books, the Dark mark is described as green. However, in the films, it was only green at the Quidditch world cup. In all other film appearances it has been grey.