Jomungandr, an immense sea serpent, is the child of Loki and the fire-giantess Angrboda, and the brother of the death goddess Hel and the wolf Fenrir. Like the rest of his family, he is destined to play a key role in Ragnarok, the apocalyptic final battle, where he will be the foe of Thor.
Odin quickly recognized the serpent would be unstoppably powerful if kept in Asgard or Jotunheim, so he flung Jormungandr into Midgard (our world) shortly after his birth. In Midgard, our world.
Jormungandr is not intelligent or capable of speech like Fenrir and Hel. What he lacks in cunning, he makes up for in sheer size; he has grown so large that he encircles the entire Earth, leaving nothing for him to eat but his own tail. In this sense, Jormungandr is a classic example of an ouroboros - a snake which devours itself.
During Ragnarok, Jormungandr will uncoil to wreak havoc on the world. Thor will sacrifice himself slaying the beast. The hammer god will fall to Jormungandr's horrible poisonous spittle, but not before landing a fatal blow with Mjolnir.
Other examples of Jormungandr's might come from stories where Thor encounters Jormungandr while fishing, and is unable to conquer him, making Jormungandr Thor's only serious rival. In Thor and Loki's adventure in Utgard, Thor is made to try and lift Jormungandr (who is disguised as a cat), and he is unable to do so.