Norse mythology

Norse or Scandinavian mythology is the belief and legends of the Scandinavian people. Norse mythology is a version of the older Germanic mythology and was later replaced by Christianity for the most part.
Norse mythology is a set of beliefs and stories shared by Northern Germanic tribes. It was not handed down from the gods to the mortal. It had no scripture. The mythology was passed on from one generation to the next in the form of poetry. It continued to be passed down this way through the time of the Vikings. The original beliefs were long lost. Our knowledge about it is mainly based on the Eddas and other medieval texts. These were written down while and after they turned to Christianity.
Cosmology
In Norse mythology, the universe was thought to have 9 realms or "worlds". Asgard is where the gods lived. Asgard could only be reached by walking across the rainbow (the Bifrost bridge). The Frost Giants lived in a place called Jotunheimr. Jotunheimr means giant realm.
A cold, dark place called Niflheim was ruled by Hel. She was the daughter of Loki. This was the eventual home of most of the dead. Located somewhere in the south was the fiery realm of Muspelheim, home of the fire giants.
In between Asgard and Niflheim was Midgard, the world of men.
Supernatural beings
There are three "clans" of deities, the AEsir, the Vanir, and the Jotnar (referred to as giants in this article). After a long war, the AEsir and Vanir made peace and joined together.
The AEsir and the Vanir are enemies with the Jotnar or giants. The AEsir are descendants of Jotnar. Both AEsir and Vanir intermarry with them. There are two kinds of giant: frost-giants and fire-giants.
There are many other supernatural beings. These include:
Fenrir the gigantic wolf
Jormungandr the sea-serpent that is coiled around the world.
Hugin and Munin (thought and memory), the two ravens who keep Odin informed of what is happening on earth.
Ratatoskr, the squirrel which scampers in the branches of the world tree, Yggdrasil.
Sources
Most of this mythology was passed down orally as Skaldic poetry, and much of it has been lost. Some of it was recorded by Christian scholars. The earlier detailed records come from the Eddas and the Heimskringla by Snorri Sturluson though the mentioning of their deities And mythology goes back to Cornelius Tacitus' report "Germania" in 98 AD.
There are also several runestones and image stones that show scenes from Norse mythology, such as Thor's fishing trip and Odin being devoured by Fenrir.
Modern influences
The Germanic gods have affected elements of every day western life in most countries that speak Germanic languages. An example is some of the names of the days of the week. The days were named after Roman gods in Latin (named after Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn). The names for Tuesday through Friday were replaced with Germanic versions of the Roman gods. In English, Saturn was not replaced. Saturday is named after the sabbath in German, and is called "washing day" in Scandinavia.
Modern popular culture
J.R.R. Tolkien's The Lord of the Rings was influenced by the myths of the Northern Europeans. As it became popular, parts of its fantasy world moved into how people see the fantasy genre. In almost any modern fantasy novel, you can find Norse creatures like elves, dwarves, and giants.
Other websites
W. Wagner's "Asgard and the Home of the Gods" e-book
Peter Andreas Munch's "Norse Mythology: Legends of Gods and Heroes" e-book
heimskringla.no Old Norse Prose and Poetry
Timeless Myths - Norse Mythology - Information and tales from Norse and Germanic literatures
Norse Gods, Goddesses, Giants, Dwarves and Wights
CyberSamurai Encyclopedia of Norse Mythology