In Norse belief, the nine realms are known as Asgard, Midgard, Vanaheim, Alfheim, Svartalfheim, Niflheim, Muspelheim, Helheim, and Jotunheim. Each realm holds its unique characteristics and inhabitants, with Asgard being home to the gods and Midgard representing the realm of humans. These realms are interconnected by the Yggdrasil, the world tree that serves as a cosmic axis connecting all nine worlds.