Guinevere was King Arthur's Queen. Guinevere may be an epithet - the Welsh form Gwenhwyfar (in older spelling Gwenhwyvar) can be translated The White Fay; however, as Rachael Bromwich notes, it can also be analyzed as "Gwenhwy-vawr" or Gwen the Great in contrast to the personage "Gwenhwy-vach" -- Gwen the less. She is childless in most stories (one exception is the Perlesvaus), although there are mentions of Arthur's sons in the Welsh Triads.
The earliest mention of Guinevere was in the Welsh tale Culhwch and Olwen, where she appears as Arthur's queen, but little more is said about her. Caradog of Llancarfan, who wrote his Life of Gildas before 1136, recounts how she was kidnapped by Melwas, king of the Summer Country, and held prisoner at his stronghold at Glastonbury. The story states that Arthur spent a year searching for her, found her, and had assembled an army to storm Melwas' fort when Saint Gildas negotiated a peaceful resolution and restored Guinevere to Arthur. The Welsh poet Dafydd ap Gwilym alludes to this story in two different poems. The medievalist R.S. Loomis suggested that this tale of her abduction seems "to show that she had inherited the role of a Celtic Persephone".
Geoffrey of Monmouth tells a slightly different version of Guinevere's abduction, adding that she was descended from a noble Roman family and was the ward of Cador, Duke of Cornwall. Arthur left her in the care of his nephew Mordred while he crossed over to Europe to go to war with the (fictitious) Procurator of Rome Lucius Hiberius. While he was absent, Mordred seduced Guinevere, declared himself king and took her as his own queen; this forced Arthur to return to Britain, and fought Mordred at the Battle of Camlann.
Chretien de Troyes tells yet another version of Guinevere's abduction, this time by Meleagant (whose name can be shown to be derived from Melwas). But instead of Arthur being Guinevere's rescuer, Chretien introduces Lancelot to the story, who sets off with his cousin Gawain to rescue her in Chretien's epic poem of the same name. It is this version that becomes favored in later accounts, as for example the version scholars have called The Vulgate Cycle. In the later adaptations she is described as the daughter of King Leodegrance, and is betrothed to Arthur early in his career, while he was garnering support. He later sent Sir Lancelot to bring her to Camelot, and although Guinevere and Lancelot fell in love on the return journey, upon reaching Camelot she fulfilled her duty and married Arthur -- yet continuing their affair.
There is also a 1994 motion picture about the character of legend. Guinevere is also a 1999 motion picture that had nothing to do with the King Arthur character above.