An Unbreakable Vow is an oath that one person makes to another. If that person breaks the Unbreakable Vow, they die. The Vow is made by the two people joining right hands and a third person placing the tip of his or her wand on their hands, whereupon the person the Vow is being made to asks the other person if he or she will perform certain actions and the other person responds with "I will."
Severus Snape made an Unbreakable Vow in Harry Potter and the Half-Blood Prince to Narcissa Malfoy to "watch over…Draco, as he attempts to fulfill the Dark Lord's wishes…protect him from harm…and…if it seems Draco will fail…[to] carry out the deed that the Dark Lord has ordered Draco to perform."[HP6]
According to Ron Weasley, his brothers Fred and George tried to get him to make an Unbreakable Vow when he was a young child. Ron comments that it was the only time he saw his father get as angry as his mother,[HP6] other than (possibly) his father's argument with Percy in 1995.[HP5]