It's the religion of the tribal people in Iceland, before Christianity became the dominant religion. Ironically, most of the records of the mythology that some tribes might have believed in, was recorded by a Christian missionary in Iceland, Snorri Sturluson, because I guess he thought that it was interesting.
So, he at least did pagans the favor of preserving some knowledge of the Eddas, wherein recounts the creation myth, the existence of jotnar (giants), and the role of the Aesir: Odin, who's basically like the administrator; Thor, Loki, Freya, Hela, Frey, Tyr...all the gods.
The mythology is usually sourced from the prose and poetic Eddas, as well as compilations of the Icelandic Sagas. I think Jacob Grimm (of the Grimm brothers fame) also wrote up something on Teutonic Mythology.
Practice...huh, my source is/was osmosis with practicing Norse Reconstructionists, so...ask them? Right here: http://www.spellsofmagic.com/read_topic.html?topic=262