Queen Guinevere is often described as one of the fairest women in the country and is often the daughter of some form of royalty. She is the wife of King Arthur and in legend her marriage dowry is the Round Table. She also has an affair with Sir Lancelot, one of King Arthur's knights. (1)

Queen Guinevere was seen as one of the most beautiful women in history. She had an affair with Sir Lancelot. She was also used by many of the knights of the Round Table even King Arthur. Sir Lancelot helped her escape many times. After the death of her husband King Arthur she fled to Amesbury where she became a nun.
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Origin of Guinevere

Guinevere first appeared in Geoffrey of Monmouth’s version of King Arthur, which was the first full account of the legend. In his account, Guinevere was a descendent of a noble Roman family. Duke Cador welcomed Guinevere into his household and brought her up. However it doesn’t explain how Guinevere found her way to the Cornish family.

Other writers that expanded on her story say that her father (Leodegan) was king of Carmeldie/Camelered and that he owned the Round Table before Arthur. Merlin had made the Round Table for Uther and Uther gave it to Leodegan. When Arthur married Guinevere, the Round Table was given to him as a part of her dowry.
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Sources:
1) http://www.arthurian-legend.com/more-about/more-about-arthur-9.php
2)http://csis.pace.edu/grendel/projs993a/arthurian/guinevere.htm
3)http://www.camelotintl.com/legend/guinever.html