In Norse mythology there are 'nine worlds' (níu heimar), that many scholars summarize as follows:
world of average human experience
world of the Álfar (elves).
world of the Svartálfar (black elves)
world of the Vanir
world of fire
world of the jötnar
world of ice
world of the Æsir
world of the Niðavellir, netherworld.
Each world also had significant places within. Valhalla is Odin's hall located in Asgard. It was also home of the Einherjar, who were the souls of the greatest warriors. These warriors were selected by the Valkyries. The Einherjar would help defend the gods during Ragnarok.
These worlds are connected by Yggdrasil, the world tree, a giant tree with Asgard at its top. Chewing at its roots in Niflheim is Nidhogg, a ferocious serpent or dragon. Asgard can also be reached by Bifrost, a rainbow bridge guarded by Heimdall, a god who can see and hear a thousand miles