The Asgardians refer to all of the major known inhabited realms of their cosmology as the "Nine Worlds". Only four of the Nine Worlds are located on the main Asgardian landmass: Asgard, home of the Gods, Vanaheim, home of the Asgardian's sister race, the Vanir, Nidavellir, home of the Dwarves, and Alfheim, home of the Light Elves.
The remainder of the Nine Worlds are on separate landmasses isolated from one another by interdimensional space. (For the sake of creating a comprehensible diagram, the Nine Worlds of Asgard are placed in a multi-leveled configuration. These levels do not represent any real physical distances or relationships between the realms.). Midgard, our Earth, does not appear to be physically affected by the motions of any of the other physical bodies in the Asgardian cosmology, although Earth's axis (the imaginary pole around which it rotates) is in alignment with one of the roots of Yggdrasil, the comsic ash tree that stands in Asgard. Jotunheim, the world of the giants, is a flat ring-shaped realm with high mountains along its inner edge. It is apparently on its own separate dimension plane, discrete from Asgard's and Earth's. Svartalfheim, home of the Dark Elves, is another asteroid-like landmass, small than Asgard. There are numerous nexus-portals between the mountains of Asgard permitting easy passage by denizens of each realm. These passageways make Jotunheim and Svartalfheim seem like "underworlds" of the Asgardian continent itself.
The eighth of the Nine Worlds is Hel, realm of the dead, and its sister realm, Niffleheim. In the Asgardian scheme of afterlife, the heroes and honored dead go to Valhalla, a special region of Asgard, the common dead go to Hel, and the dishonored dead (murderers and other evil-doers) go to Niffleheim. Hel, Niffleheim, and Valhalla possess the necessary conditions to permit the astral forms of the deceased to exists there for indefinite periods of time. At one time, Hela, goddess of the dead, usurped the rule of Valhalla, despite the fact it was on a different dimension plane than Hel. Odin has since reclaimed the land. The ninth of the Nine Worlds is Muspelheim, land of the fiery demons. Until his recent disappearance, the primordial demon Surtur ruled Muspelheim. Muspelheim is on its won dimensional plane, separate from all the other Nine Worlds. In the Asgardians' account of the origin of their cosmology, Muspelheim, the land of fire, and Niffleheim, the land of ice, were said to predate recorded time, separated from one another by Ginnungagap, the Yawning Void.
Asgard is honeycombed with nexus-portals to the variuos extradimensional realms of the Nine Worlds, making the worlds (with the exception of Earth) sometimes seems like they are on a contiguous plain. (Indeed, early cartographers of Asgard mapped it this way). The only permanent portal to Earth is Bifrost, the Rainbow Bridge. There is a special passageway from Asgard to the extra-dimensional realm of Olympus, home of the Greek gods. Since Olympus is not a part of the Asgardian cosmology, this nexus-portal is believed to be an artificial rather than natural phenomenon. Another unique feature of Asgard is the Cave of Time, an apparently natural phenomenon through which passage to other time eras is possible.
It is probable that somewhere in Asgard's outlying Sea of Space there are floating nexus-portals to Earth's space. Beings of the Earthly dimension have in certain instances been able to travel from Earthly to Asgardian space. There may indeed be an edge to Asgard's Sea of Space, perhaps at the perimeter of the "Dome of the Sky" extending from the outer edge of ringed Jotunheim. At the edge of the Sea there would exist "dimensional borderlands" which serve as transitional areas between discrete dimensions.
Asgard and its sister realms aer populated by six distinct humanoid races. Besides these, there are several singular creatures who exists upon various of the Nine Worlds. The first is the Midgard Serpent Jormungand, an immense snake-like dragon whose body encircles the inner edge of the mountains of Jotunheim closest to Midgard. The Midgard Serpent lies at the very edge of the dimensional boundary between Jotunheim and Midgard, and prevents passage between dimensions. The second is the winged creature Nidhogg, who lives in Niffleheim and gnaws at one of the roots of Yggdrasil. The third is the giant wolf-god Fenris, who is responsible for the war-god Tyr's loss of a hand. Fenris is imprisoned in the distant land of Varinheim. The fourth creature is the primordial ice giant Ymir.