The jötunn (devourer; plural jötnar), also referred to as ettin, thurs, or giants (risar in Old Norse), are primordial forces in Norse mythology that represent chaos. Although not inherently evil, their chaotic nature is a threat to the order of the gods which is needed for the worlds of Yggdrasil to be.
The Jötunn were the first beings, their ancestral father being Ymir, who was killed by the sons of Bór, Odin, Vili, and Ve, to create the world. Most of the Jötunn drowned in Ymir's blood, but some survived to repopulate the Jötunn again. The gods built Yggdrasil out of Ymir's body and founded Jötunheimr (Jötunn Home) where the giants could live, cut off from the other worlds, especially Midgard.
Some of the Jötunar are allies, friends, and even lovers of the gods. Loki, Týr, and Skadi are giants that have joined the gods. Mimr is Odin's mentor. Gerd is the wife and close friend of Freyr. And Ägir is a dear friend of the gods. There are still sinister giants, ones who seek to wreak havoc such as Thrym, Útgarða-Loki, Skyrmir, and Thjazi.
The Fire Giants
In Muspelheim (Home of the World Destroyers) dwells a more sinister kind of giant: the fire giants. They are led by Surtr and await the end of the world when they shall attack the gods and Surtr shall destroy all of creation with his fire. Among the fire giants are Loge and Eldr.