The Vanir (Old Norse Vanir, pronounced “VAN-ear”) are one of the two principal tribes of deities featured in Norse mythology. (The other tribe is the Aesir.) Among their ranks are Freya, Freyr, Njord, and arguably the early Germanic goddess Nerthus as well. Their home is Vanaheim, one of the Nine Worlds held within the branches of the world-tree Yggdrasil.
–from Norse Mythology for Smart People by Daniel McCoy
While the Aesir were known for their warlike nature and fighting ability, the Vanir were known for their powers over the sea and the fertility of the land. Why the two groups were so much at odds with one another isn’t clear since cooperating seems a sensible solution for both groups to be protected (through the powers of the Aesir) and able to propagate and fill the land (through the powers of the Vanir). But never mind. Eventually both sides recognized the strength of the other and decided continuing battles were counterproductive.
Thus ended the first Aesir-Vanir War.
About the only Vanir recorded in the Poetic Edda are those mentioned by Daniel McCoy in the above introductory paragraph. The Vanir gave Njord and his twin children, Freyr and Freyja, as hostages to the Aesir as guarantee of their good will at the end of the war in exchange for the pair Hoenir and Mimir given by the Aesir to the Vanir. From that point on, the three Vanir gods become integrated into the world of the Aesir. So much so that it is not clear whether Freyja and her husband Odr are not in fact Frigg and Odin.
And then there is Nerthus, not mentioned in the Poetic Edda at all. Roman historian Tacitus in his Germania (authored about 100 CE) referred to her as Mother Earth and described rituals where she would arrive in an area riding in a chariot drawn by cattle, accompanied by priests. Before her arrival, all iron implements must be hidden away so that there will be no war or violence while she is present. While she is there, the populace make merry and celebrate until she decides it is time for her to move on. Once she leaves an area, the chariot and all vestments as well as Nerthus herself are taken to a lake to be spiritually cleansed. All those humans involved in the cleansing are then drowned in the lake, apparently willing sacrifices for the goodness Nerthus brings to the earth. 
But it isn’t that simple. Those with more linguistic knowledge than I have pointed out that the name Njord is exactly what the Proto-Germanic name Nerthus would look like if translated into Old Norse. Perhaps Nerthus and Njord are really a linked pair, like Freyr and Freya, or even that they are names for the two aspects of a hermaphroditic god. 
 McCoy, Daniel. Norse Mythology for Smart People, article on Nerthus.
Featured image credit: “Njord’s Desire of the Sea” by W.G. Collingwood (1908)