The Nine Worlds of the Norse Cosmos
Odin and his brothers slew the giant Ymir and made Yggdrasil and several worlds from his body. There are nine worlds total. Múspellsheimr, Niflheimr, Jötunheimr, Miðgarðr, Ásgarðr, Svartálfaheimr, Ljossálfaheimr, Vanaheimr, and Helheimr are the worlds of Yggdrasil.
In the realm of fire, Múspellsheimr , there was an inhabitant who has more years on the gods and giants of all the worlds. He has existed since the beginning of time, and his origins are unknown this being is Surtr. He wields a giant fiery sword and in the end will slay Freyr and lay waste to the nine worlds on Ragnarök. Other than that it is home to fire giants for the most part.
Niflheimr is the primal ancient home of the frost giants. It is icy, barren, and inhospitable yet the surly frost etins dwell there. Frost etins are the sworn enemy of Thor, and many of them do not appear to be friendly towards humanity.
Jötunheimr was created by the gods so that the Jötunar (giants) could have a peaceable place to dwell. It is certainly a rocky terrain having been made from Ymir’s bones, but it is still fairly habitable for the giants. Notable residents are Utgard-Loki as well as many giants mentioned by name in the Eddas.
Miðgarðr is the home of man. We humans dwell here along with all other life that lives upon Miðgarðr. We share this world with other animals as well as Landvættir (land spirits), Dökkálfar, and other manner of wights and sentient beings. It is a diverse and blessed world.
Thor watches over Miðgarðr and wards all evils from us. All the other gods shine gifts upon Miðgarðr as they see fit and bless the people of Miðgarðr for honoring them.
Ásgarðr is the home of the gods. It is the highest realm in Yggdrasil. Odin, the Æsir, some Vanir, and the honored dead dwell in Ásgarðr. It is a sacred world unmatched in beauty and prestige.
It is home to the halls Valhöll, Bilskirnir, Sessrúmnir, and many other dwelling of the gods and Einherjar (honored dead of Miðgarðr.)
Svartálfaheimr is home to the Svartálfar and duergar (dwarves.) It is a dark and bleak realm. The soot of the dwarf forges covers most things and the inhabitants of Svartálfaheimr do not mix well with sunlight.
Ljossálfaheimr is a different story. It is a home of light and the realm governed by Freyr. As such it is a holy realm. The light elves live there and they are semi-divine beings who are usually indifferent to humanity and rarely harmful.
Vanaheimr is a fertile and gentle land of the Vanir. Njörðr dwells here along with some members of the Æsir and the rest of the Vanir tribe of gods. It is said to be warm and have calm seas and sea side halls and dwellings.
Helheimr is the underworld and is ruled by the goddess of death and decay, Hel. She is not really evil per se but rather dark and gloomy, so gloomy that the gods gave her the realm of Helheimr to take care of because they could not bear to see her. The wyrm Níðhöggr is said to feast on the bodies of dead murders, criminals, and oath breakers here on the Shores of Náströnd. It is also home to the souls of Miðgarðr that were not worthy of Ásgarðr.