The Dark Mark is the sign of Voldemort. Conjured by the incantation Morsmordre, it takes the form of a giant skull, composed of what looks like emerald stars, with a serpent protruding from its mouth like a tongue.
The Dark Mark is conjured in the woods by the campsite amid the chaos after the Quidditch World Cup. It hovers above the woods, illuminating the whole area like a grisly neon sign. The appearance of the Dark Mark causes the entire wood to erupt in screams, striking terror in those who see it. Before the events at the campsite, the Dark Mark hadn't been seen for thirteen years, since the day Voldemort disappeared. Voldemort and his followers used to send the Dark Mark into the air whenever they had killed. To see the Dark Mark hovering over your house was everyone's worst fear, as they knew what they were about to find inside.
The Death Eaters, Voldemort's followers, fled the moment they saw the Dark Mark in the air. Barty Crouch's house-elf, Winky, is found unconscious in the spot Harry, Ron and Hermione heard the spell being cast.
The Dark Mark was burnt into the forearm of each Death Eater by Lord Voldemort when they first joined him, as a means of distinguishing each other, and as a way for him to summon them to him. The Mark on both Snape and Karkaroff has been growing clearer all year, and burns black when he summons them to his side.