The Æsir or Aesir are the principal gods of the Norse pantheon. They are one of two major clans of Norse deities with the other being the Vanir. Among the Aesir are many famous figures found in the Scandinavian tales such as Odin, Frigg, Thor, Baldr and Tyr. These Norse gods are understood to dwell in Asgard, a realm separated from the mortal world (Midgard) by Bifrost (the rainbow bridge).
The distinction between Aesir and Vanir is relative, for the two are said to have made peace, exchanged hostages, intermarried and reigned together after a prolonged war. In fact, the most significant divergence between the two groups is in their respective areas of influence, with the Aesir representing war and conquest, and the Vanir representing exploration, fertility and wealth.The Jotun, on the other hand, are seen as a generally malefic (though wise) race of giants who represented the primary adversaries of the Aesir and Vanir.
The Æsir, though immortal, were somewhat more "perishable" than their Indo-European brethren. Not only was their eternal youth maintained artificially (through the consumption of Iðunn's golden apples), they could also be slain as evidenced in their demise during Ragnarök.