Suggested reading: Norse Mythology
In Norse Mythology, the Jotuns are the giants who were the oldest living creature, older than even the gods. The oldest of the Jotuns was Ymir, who was killed by Odin and his two brothers (Vili and Vei). Almost all of the Jotuns drowned in Ymir's blood, except for one, from which all of the other Jotuns come from (one must wonder how...).
The Jotuns are the main cause of conflict in Norse Mythology, they are the ones who when they ally with Surtr, will cause Ragnarok and Bifrost to fall. Although the gods are all powerful, there is only one god that is able to kill the Jotuns, and that god is Thor. Even though the Jotuns are the enemies of the Aesir, Odin often goes to them for wisdom, because some of them are considered very wise.
The Jotuns were said to be around 12 feet tall and have a similar resemblance to a man, but much stronger than any man. They were also very ugly, some so ugly that they no longer looked or had any resemblance to man. The female Jotun, called the Jotunmeyjar, was also quite ugly. Except for a few of them, who were very beautiful. They were so beautiful, that some of the Gods fell in love with them.
The gods, all respecting and kind, despite being deep foes with the Jotuns, gave them homes outside of Midgard. But not to close, because they were determined to keep them from harming man...