Jotunheim is one of the Nine Realms and the home of the Frost Giants. The planet is seen to be deteriorating from the lack of ice magic that comes from the Casket of Ancient Winters. It was ruled by Laufey.
In 965 A.D., Jotunheim was the setting of the final battle between the Frost Giants and the Asgardians during the Asgard-Jotunheim War. After the battle, Odin took the Casket of Ancient Winters, depriving the Frost Giants of their power, and Laufey's son, who was left to die by his own father. Without the Casket, the realm began to decay and break apart so that the full force of the Bifrost could rip it apart.
Thor and his friends came to Jotunheim for some answers on their recent break-in to Asgard. The Asgardians encountered a fight, where they fought a seemingly endless army of Frost Giants along with an enormous beast. Odin came on Sleipnir to rescue his subjects, and returned them to Asgard.
Loki later takes a visit to Jotunheim to strike a deal with Laufey. He would allow Laufey to go to Asgard to kill Odin. That way, Laufey's worst enemy would be killed and Loki would take the throne. However, Loki betrays and kills Laufey, his father. He then overloads the Bifrost Bridge's power in order to destroy the monster-full Jotunheim. However, Thor destroys the Rainbow Bridge before the realm could be completely destroyed.