Æsir and Vanir
There are 2 pantheons of gods in Norse mythology, the Æsir and the Vanir. The term Æsir simply means ‘gods’ in Old Icelandic, but its Indo-European root, *h₂énsus, means ‘breath’, suggesting the links between deities and life-giving. ‘Vanir’ is the etymological root of the Icelandic word for friend, though the precise origin of the name for the gods is uncertain. Unusually amongst world religions, the 2 pantheons do not displace one another, but live contemporarily, by contrast to the Titans and Olympians in Greek Mythology, for example. The pantheons are merged after fighting a war early in the universe’s history.
The Æsir are the principal Norse gods, including Odin, Thor, Tyr, and Baldr. Amongst the Vanir, the most notable members are Njörðr, Freyr and Freyja. The Vanir tend to have a stronger relation to fertility, wisdom, and soothsaying, but to understand best the differences between the groups it is best to consider the Æsir -Vanir War. This took place at the beginning of the world, and was brought about by the magic practiced by the Vanir. Our chief sources for this conflict are, again, Snorri, who discusses it in both the Prose Edda and Ynlinga saga, and the Poetic Edda.
The war starts when Gullveig enters Asgard to practice seiðr, a form of divination and manipulative sorcery. At this stage, only the Vanir practiced seiðr, and it seems that its introduction to Asgard created havoc:
She charmed them with spells;
She made magic wherever she could, with magic she played with minds,
She was always the favourite of evil women. (Voluspa, 22)
Being the ‘favourite of evil women’, in its historical context, probably means some sort of sex-magic. The name Gullveig itself means ‘gold-intoxication’, which suggests that she introduced seiðr amongst the Æsir out of greed for wealth.
The Æsir’s reaction to Gullveig was uncompromising, as the Poetic Edda tells us:
They buttressed Gullveig with spears
And in One-eye’s [Odin’s] hall they burned her;
Three times they burned her, three times she was reborn. (Voluspa, 21)
Seiðr lies behind Gullveig’s miraculous reincarnation, which must have angered the Æsir yet more. Thus the first war broke out, with Odin throwing the first spear. In Ynglinga saga Snorri elaborates that the war was an even contest in which neither side could gain the advantage, and thus a truce was called and hostages exchanged, leading to the merging of the pantheons.