In Norse mythology, the Vanir were a group of gods and goddesses associated with fertility, prosperity and eroticism. They were direct descendants of Holde, the crone goddess, and were considered to be the givers of wealth, luck, health, fertility and youth. The Vanir were the masters of magic. Unlike the younger warrior gods of the Aesir, the deities of the Vanir were peaceable and gentle, more concerned with nature than dominance. They resided in Vanaheim (also known as Vanaheimr or Vanaland) which is never described, only spoken of in the Alivssmal.
Little is known of the origin of the Vanir, though some people believe that the deities represent an older pantheon of the people encountered by invading Indo-Europeans. Or, they may have just been a part of the typical Indo-European pattern of gods and goddesses arranged into different factions who perform specific types of functions, such as military or religious. Under this arrangement, the Aesir are the military and religious deities, while the Vanir are the producers. The lack of information regarding the heritage of the Vanir could indicate that their function was simply to provide counterpoint to the Aesir.
The war between the Aesir and the Vanir (which may be representative of encounters between Germanic peoples and indigenous cultures) began when Odin and Thor denied full godhood status to the Vanir. They responded by sending the Aesir a woman, Gullveig (gold-drink, gold drunkenness), who corrupted them. War broke out, and only ended when both groups were too exhausted to go on. At that point, the gods on both sides pledged a truce, which was solidified with an exchange of members of each side. The Vanir sent Njord with his children twin Freya and Freyr, and in return, the Aesir sent Hoenir and the wise Mimir. The Vanir were angered, however, when they realized that Hoenir was nothing more than a puppet for Mimir’s words, and so they sent Odin the head of Mimir. That action precipitated no further malice on either side, and from that point on, the two groups peacefully coexisted. Njord, Freyr and Freya actually went on to become popular Aesir deities.
To celebrate the truce, the gods of both the Vanir and the Aesir chewed berries and spit them into a bowl, creating Kvasir (a giant who represented peace among the gods). He was later sacrificed so that the dwarves could make a more potent drink from his blood to inebriate the gods and inspire the poets.
The gods of the Vanir are:
Holde
Nerthus
Njord
Freya
Freyr
Od
Hnoss
Aegir
Ran
Ullr
Ulla
Gerdh
Skirnir
Heimdallr
Idunna
Bragi
Siofyn
Gefjon
Skadhi
Erde
the Undines
Svol
Ostara
Gullveig
The name Vanir might have come from the old-Norse world 'vinr' which means friend.
Sources:
http://www.mirthe.org/mythology/norse.php
http://www.pantheon.org/articles/v/vanir.html
http://www.traditionalvisionpagan.org/gods_goddesses.html