Who was Guinevere ?

Guinevere is Arthur's wife and queen; according to the Vulgate Cycle and Malory, she is the daughter of Leodegrance of Carmelide. Though one of the Welsh Triads (Triad 56) speaks of Arthur's three great queens (all named Gwenhwyfar), later romance generally gives him only one wife named Guinevere. 

Geoffrey of Monmouth introduces the notion of Guinevere's infidelity (with Modred) while Arthur is fighting on the continent. In Chrétien's Lancelot, Guinevere becomes Lancelot's lover after he rescues her from Meleagant. Generally (though not always) in the romance tradition, Guinevere is portrayed as Lancelot's lover. In the Vulgate Cycle, the first meeting between Guinevere and Lancelot is arranged by Galehaut. She is later accused of not being the true Guinevere by the illegitimate daughter of her father Leodagan and the wife of his seneschal. When Arthur falls. in love with the False Guinevere and accepts her as his queen, Guinevere is protected by Lancelot and Galehaut until the truth is revealed. 

Malory's Guinevere is jealous and demanding but also a true lover. Her jealousy and anger drive Lancelot mad and lead her to say she wishes he were dead. Nevertheless, she remains true to him. She is accused several times of crimes-infidelity and the murder of Mador's relative-and must be saved by Lancelot, as she is once again when their love is discovered and she is'sentenced to be burned at the stake. When Mordred rebels against Arthur and attempts to marry her, she flees first to the Tower of London and then to the nunnery at Amesbury, where she becomes abbess. Lancelot visits her there after the death of Arthur, but she asks him to leave and never to return and refuses even to give him a final kiss. She dies a holy death, of which Lancelot learns in a vision that instructs him to have her buried next to Arthur. 

While Malory understands of the true love of Guinevere, Tennyson makes her an example of an unfaithful wife.-Arthur, before whom she grovels with guilt when he visits her in the nunnery, says that she has "spoilt the purpose of my life." Malory also writes that Arthur had an affair with Lyonors, the daughter of Earl Sanam. Lyonors bears him a  son named Borre, who later becomes a knight of the Round Table.