The universe has nine planes divided into three levels within Yggdrasil, the World Tree. In some sense the World Tree is like the Kabalistic Tree of Life for it is here that all the gods, forces, and their relations to manifested life and being are found and are open to our study and understanding.
The upper level consists of
Asgard: Home of the Aesir - gods of power and war; opponents of the Vanir.
Alfheim: Home of the elves
Vanaheim: Home of the Vanir – gods of chaos, fertility, and cultivation; opponents of the Aesir
The Middle Level of
Midgard: Home of humanity
Jotunheim: the first created world, Home of Mimir and the giants
Svartalfaheim: Home of the dark-elves, trolls, and gnomes
Nithavellir: Dark Field, home of the dwarves
The Lower Level of
Muspelheim: Home to the fire demons
Niflheim: Land of Mists and the Dead, ruled by Hel. It is the realm of the instincts, inertia, and the unconscious.
Helgrind (Death Gate) separates the land of the living from the land of the dead.
Bifröst, the Rainbow Bridge from Heaven to Earth
The Gods built the rainbow bridge from Asgard (heaven) to Midgard (earth). The divine forces ride daily over the great bridge to mingle with men. As strong as Bifröst is, it will collapse when the Frost Giants ride out over it at Ragnarok. All is chaos when the Gods and Giants are at war.
After the Bifrost Rainbow Bridge collapses and one by one each of the Worlds will fall. The remaining Aesir will escape in Freyr's ship, Skidbladnir. It will be almost overtaken by the Hel-ship Naglfar. Midgard will then be destroyed by fire and will sink into the sea.
The universe ends in a fire storm and a new cosmos will arise from the ashes of the old world. The Earth reemerging from the sea. Seven sons of the dead Aesir will return to Asgard and rule the universe.. A new generation of gods and humans will arise to dwell in harmony.
Source: Slate, J. & Weschcke, C.: The Llewellyn Complete Book of Psychic Empowerment – Tools & Techniques, 2011, Llewellyn.