The Vanir are the second family of gods. The other family are the Æsir.
Primarily, all gods are called Æsir, but some of them, Njǫrðr, Freyr, Freyja, Ing and maybe Ullr, belong to the Vanir.
They are in particular fertility gods and they practised a form of magic deemed to be ignoble by the Æsir and they live in Vanaheimr.