The Dark Mark is a mark left by Voldemort or a Death Eater when ever they kill a person. It is shown in the sky after a death, and Arthur Weasley said that seeing it above a house is everyone's Worst fear, as they know whats inside. The first mention of the Dark Mark is in Harry Potter and the Goblet of Fire when Hermione tells Harry what the symbol is in the sky is, after Barty Crouch Jr used it.
The Dark Mark is a skull with a snake coming from its mouth. On an arm the dark mark appears black, but when cast using Morsmordre it is green.
 Dark Mark on Arm
The tattoo has a special link to Lord Voldemort allowing him to summon his followers when it is touched by his wand. It can be used to signal Voldemort by touching the mark with their own hands. When touched the tattoo seems to move. The dark mark goes faint when inactive, and when it was touched by Voldemort, it goes to a Jet black colour. The mark delivers a burning sensation to anyone with the dark mark, and could be used to signal Voldemort by pressing on the mark. The mark was only given to Voldemort's inner circle, so it was considered an honour to receive it.
 Dark Mark in the Sky
This was cast by saying the incantation Morsmordre, this mark is only seen twice in the films, once being during the Quidditch World Cup in Harry Potter and the Goblet of Fire and the other being over the Astronomy Tower at the murder of Dumbledore in Harry Potter and the Half Blood Prince.