Jormungand is the old Norse name for the Midgard Serpent, a monster of ancient Norse mythology. Rare among monsters in that it supposedly lived in Midgard, the realm of Humans (most monsters of Norse mythology dwelled in Udgard, along with the evil giants who plotted against Gods and men alike). Jormungand was a gigantic serpent, which dwelled in the great sea that the Vikings believed surrounded all of the human world. The enormous beast encircled all of Midgard, so a careless sea explorer would certainly meet the Serpent if he ventured too far from land. Needless to say, an encounter with the Serpent was supposedly not a Good Thing at all.
Jormungand was the second child of Loki, the God of Deception, and a giantess named Angraboda. Considering that the two others were Fenris (a wolf monster) and Hel (who would later become the ruler of the Underworld), and that Loki was also the father (or, rather, mother -- he had assumed the form of a mare at the time) of Sleipnir, Odin's eight-legged horse, you could imagine that Loki's family reunions would be interesting settings, to say the least.
Thor, the Norse god of thunder, wasn't terribly fond of Jormungand. As myth would have it, whenever fishermen, sea explorers or Viking raiding parties encountered the Serpent, Thor would often come along to fight the great snake. It is possible that this particular myth arose because of thunderstorms at sea (the Norsemen believed that thunderstorms were caused by Thor riding across the sky in his goat-pulled chariot), where the ocean's waves ripped at ships like they were a gigantic snake. Ancient Norsemen spent a considerable amount of their time at sea, so many of their myths and legends revolve around such matters.
Thor attempted to kill Jormangund on several occasions, the most often-cited being an ill-fated fishing trip conducted by him and the frost giant Hymir. The two set out to catch the serpent, using an ox head for bait. The serpent took the bait, but the resulting struggle was too much for Hymir, who feared for his life and cut the fishing line. Jormungand escaped, angrier than ever.
During Ragnarok, Norse mythology's Armageddon, Jormungand rises from the oceans to participate in the final battle between good and evil, Gods and Giants. You can probably guess who it seeks out to fight. After an epic battle Thor finally kills the serpent, but dies shortly afterwards from Jormungand's venom.