Sif is a goddess that was associated with the domains of family and fertility. Her most distinguishing feature is her long golden hair, which has been connected by scholars to golden fields of wheat.
She is speculated to be a daughter of Odin. This makes her both the sister and wife of Thor, with whom she had a daughter, Thrud. She also has a son from her first marriage, Ullr. Her first marriage was to an unknown husband, most likely a giant or the Vanir god Njörd.
The most famous story in which she participates is the one in which her head is shaven by Loki. Her story gets a happy ending when the famous dwarf smiths, named the sons of Ivaldi, create a magical golden hairpiece for her that grows likes real hair.
In another story, she is accused of cheating on her husband by Loki. We are not sure if we want to emphasize that insult too much, as Loki was rather drunk when he made these accusations.