Freyja is a Norse goddess associated with love, sex, beauty, fertility, gold, seiðr, war, and death. After the war between the Aesir and the Vanir Freyja and her brother Freyr joined the Aesir in the realm of Asgard. Most notably, Freyja is known for riding a chariot pulled by two cats. The cats are named Bygul (Bee gold, or Honey) and Trjegul (Tree gold, or Amber), both names befitting the goddess of gold.
Freyja is a very interesting character in the Mythology. Living among the Aesir she is both treated as an equal while simultaneously being used as a bargaining chip by her fellow gods and goddesses. It's no surprise that Loki is a large part of Freyja's sorrows.
Freyja once came upon dwarves crafting a beautiful necklace. Being obsessed with gold and beauty she could not leave it. She begged and pleaded with the dwarves to let her purchase it. They refused all her offers of treasure and wealth. When Freyja finally asked what the necklace would cost the dwarves responded "The treasure of your love." and so she laid with each of the dwarves to receive her prize.
Loki learned of the exchange, as Loki often knew of things he could use to cause mischief, and with the information he went to Freyja's husband and father of two daughters, Odur.
Upon hearing of this betrayal, Odur demanded proof from Loki since he had not seen the necklace which Freyja had so wisely hidden from him. Not wanting to miss the chance to cause trouble, Loki set forth to bring the necklace to Odur. He took the form of a fly (much like he did in the myth of Mjolnir's creation) and bit Freyja on the cheek as she slept so that she would turn over and he could retrieve the necklace.
Stricken with grief at the sign of his wife's infidelity, Odur left the realm of Asgard. When Freyja awoke the next morning she found both the necklace she had traded her honor for and her husband gone. In a panic she went to the Allfather Odin to confess her transgression against her husband. Odin sent the god Heimdall to retrieve Freyja's necklace from Loki. After battling Loki in the form of a seal, Heimdall was able to return the necklace to the mourning Freyja (Pictured above)
Once the necklace was returned, Odin demanded that Freyja wear it for the rest of her days as a reminder of the time her greed and lust for beautiful jewels lead her to betray the father of her children. It is said that Freyja some times cries red-gold tears as she misses her husband and wanders the realm of Midgard (earth) in search of him under different aliases. The neck;ace, known as Brísingamen, is so entwined with her identity that she even used it to finish Thor's disguise when he attempted to retrieve Mjolnir pretending to be her.
That brings us to the story of the Jotun who stole Mjolnir to ransom for Freyja's hand in marriage.
One morning Thor awoke to his trusty hammer Mjolnir missing. Making the obvious assumption he threatened Loki with violence. Eventually Loki confessed that he had nothing to do with it. He then borrowed Freyja's feathered cloak which allowed him to transform in to a falcon and search for Thor's weapon.
Loki eventually tracked down the king of the giants, Thrym, who proudly proclaimed he had stolen Mjolnir to use as leverage against the gods to gain Freyja's hand in marriage. When Loki returned with this news the gods were shocked. Thor, being Thor, suggested that Freyja marry the giant for the sake of his precious hammer. Naturally Freyja was not a fan of the idea. Eventually it was decided that Loki would transform in to a handmaid to escort Thor, disguised as Freyja, in to the land of the giants and take the hammer back.
Needless to say this was quite the affair. Thor and Loki traveled in to the Thrym's keep and once there they dined with him until he revealed Mjolnir. Once he had the hammer in his possession again, Thor proceeded to slaughter the ice giants before returning to Asgard.
On top of a pivotal role in the Mythology, Freyja also maintains an important role in the story of the honored dead. Freyja rules over the domain known as Fólkvangr. Fólkvangr is a vast meadow where half of the dead who die in battle go to await Ragnarok. (The other half, of course, go to the hall of Valhalla.)
Freyja is certainly an interesting player on the stage of Norse Mythology. Of course this far from covers everything involving the goddess of gold, love, and death. But Asatru is often referred to as "the religion with homework". I encourage you to look up more sources listing the various myths Freyja is involved with.
1) Wikipedia - https://en.wikipedia.org/wiki/Freyja
2) Wikipedia - https://en.wikipedia.org/wiki/F%C3%B3lkvangr