The Aesir (singular áss) are the principal pantheon of gods in Norse mythology. They include many of the major figures, such as Odin, Frigg, Thor, Balder and Tyr. It should, however, be noted that a second clan of gods, the Vanir, is also mentioned in the Norse mythos; the gods Freyr and Freya are Vanir gods living among the Aesir. The Vanir were mainly connected with fertility, the Aesir with power and wars.
Collectively, all of these godlike beings are known as the Asa. As, Ase and Áss may be used to described a single male member of the Aesir.
The interaction between the Aesir and the Vanir is an interesting aspect of Norse mythology. While other cultures have had "elder" and "younger" families of gods, as with the Titanss versus the Olympianss of ancient Greece, the Aesir and Vanir were portrayed as contemporary. The two clans of gods fought battles, concluded treaties, and exchanged hostages (Frey and Freya are mentioned as such hostages). It is tempting to speculate that the interactions described as occurring between Aesir and Vanir reflect the types of interaction common to various Norse clans at the time. According to another theory, the cult of the Vanir (who are mainly connected with fertility and relatively peaceful) may be of an older date, and that of the more warlike Aesir of later origin, so the mythical war may perhaps mirror a religious conflict.
The chronology of the cults would in that case not be pictured in the myths. On the other hand, only Odin and Thor were important in both myth and cult; an áss like Ull is almost unknown in the myths, but his name is seen in a lot of geographical names, especially in Sweden, so his cult was probably quite wide-spread.
The Aesir stayed forever young by eating the golden appless of Idun. Well, not quite forever - unlike the gods of some other religions, the Aesir and Vanir are not immortal; they will die at Ragnarok, with some very few exceptions.