Norse mythology may be described as the beliefs and tales of the Scandinavian branch of Teutonic religion, together with tales of heroes in whose lives gods and mythical beings play a part. Owing to the literary material available, there are more such tales than those of related tribes, though little is known about the cult. The religion sprang from a world-view dominated by the idea of Fate.
The oldest known sources of the mythology, as preserved in the Codex Regius or Elder Edda, do not give a coherent or consistent account of the gods. They are highly allusive poems which give only fragmentary information. The more connected accounts given by Snorri Sturluson in his Prose Edda, and by later writers, were only put together long after the conversion to Christianity, and were written by authors for whom the material was interesting, not a matter of belief. Snorri, who drew partly on poems also in the Elder Edda, and partly on other sources, went to some lengths to make it clear that the stories could not be a basis for true belief, by concocting a framework for the narration, a framework which at the end is shown to be a delusion.
The creation myth as recounted by Snorri, starts with a fiery realm, Muspell, coming into being in the south, guarded by a giant called Surt, and a realm of frost, Niflheim, coming into being in the north, with an open void, Ginnungagap, between. The heat melting the frost formed the giant Ymir, out of whom, in the sweat of his sleep, formed the frost-giants. Ymir was fed by a cow, also formed from the thawing frost. The cow licking the salty ice into shape formed the first of men, whose son Bor married a daughter of the frost-giants. Their children, Odin, Vili and Ve were the first gods. They killed Ymir and made the earth from his body, the ocean from his blood (which had drowned most of the frost-giants), and the sky from his skull.
It is difficult to see where the ash Yggdrasil, the world-tree, fits into this vision, but it seems to have been an essential part of the beliefs. At its base is a pool where live the three Norns, the nordic equivalent of the Fates, and here the gods come every day to hold a court. One root goes into the realm of the gods, one into the sky and one into Niflheim. Branches and base are all being gnawed at by a variety of creatures.
The gods were not all-powerful. The myths gave them beginnings and predicted their ends. They were as subject to fate as anyone. There were three principal gods and one main goddess: Odin, the many-named and many-natured lore-master; Thor, the gods' champion; Tyr the war-god; and Frig the goddess of love. These, together with Baldr and Vidar, sons of Odin, were known as the Æsir. Another family of gods constituted the Vanir: Njord, possibly a sea-god; Frey or Freyr; and Freya, a fertility goddess. Among them was Loki, sometimes an ingenious ally, sometimes the villain of the tales. Their home was Asgard. Other gods are also mentioned, suggesting that there could be no definitive version of the religion/mythology.
In some versions Njord was the father of Freyr and Freya. Freyr seems to have been a god of fertility and prosperity. A feature of his cult may have been his image being carried round the fields in a waggon, in order to bless them. It may also have required human sacrifice. Horses and boars seem to have been sacred to him.
Parallels with other Teutonic deities
The main gods appear with variations on their names in other branches of the teutonic peoples. Odin was Wotan in Germany and Woden to the Anglo-Saxons. Thor was Donner or Thunor. Tyr was Ziu or Tiw. There is a hypothesis that the original Teutonic high god was Tiwaz, corresponding to Zeus, from whom Tyr/Ziu/Tiw devolved.
The gods had a mostly hostile relationship with the giants, who in many ways seem to be on a par with them. Dragons, dwarfs, and other mythical beings also feature prominently in the stories.
Ragnarök, the doom of the gods, is the destruction of the gods and of the earth, with Surt advancing from the south and other enemies advancing on Asgard. It is an intensely ironic, or perhaps pessimistic vision, certainly fatalistic. Despite all his knowledge Odin is simply swallowed by the wolf Fenrir; despite all his strength, Thor is poisoned by the world serpent, which he has killed; Tyr and the hound Garm kill each other; and Heimdall and Loki kill each other. Surt overwhelms the earth with fire. The primary version of the tale indicates that the destruction is followed by a rebirth and the reign of the next generation of the gods, with Baldr returning from the realm of the dead.