The brothers Odin, Vilji og Ve made the world out of the giant Ymir's body. Ymir's skull is being held by four dwarfs, in west, east, north and south, so it won't fall down upon the earth. The skull represents the sky and inside it are the sun and the moon. The sky is like a bowl standing upside down on the flat earth which is surrounded by sea. In the middle of earth there's Midgardur, inhibated by men. Around Midgard is a big wall for protection against the giants, which live along the seaside.
Above the earth is a big tree, called Askur Yggdrasils. It's crown reaches above the sky. The tree has three roots, each of which lies in a well. One well, called Mimisbrunnur, is situated in Joetunheimar, or the giants' world. One, i.e. Hvergelmir, is in Niflheimar, which is a very cold world far up in the north. The third well is called Urdarbrunnur and is in Asgardur, in heaven. Asgardur is the home of the Aesir. From Asgardur a bridge lies down to Midgardur on earth. The bridge is called Bifrost but we know it as the rainbow. Far in the south is a very hot world, called Muspell.
In Edda it says that when the wolves that chase sun and moon catch them (and eat them) the world will come to an end.