Sir Lancelot (Sir Launcelot) of the Lake

Sir Lancelot is known as both the greatest and the worst of the knights at the court of of King Arthur. Lancelot is known as Lancelot of the Lake (or Lancelot du Lac) because he was raised by Vivien, the Lady of the Lake. He carrys out various good deeds, including rescuing Queen Guinevere from Meleagant, an unsuccessful quest for the Holy Grail and the rescue of Guinevere after she is condemned to be burned to death for adultery with him..Some background on Lancelot's genealogy.
Lancelot is entirely the creation of Chretien de Troyes and does not appear in the earlier chronicles of King Arthur. Lancelot first appears in Chretien de Troyes' work "The Knight of the Cart", written in the late 1100s - in this book he is a lesser ranked knight than Sir Gawain. Chretien, however, portrays Lancelot as the knight who is most skilled in arms and chivalry of all the knights in Camelot. He becomes the king's champion, fighting challenges and undertaking quests in the king's name. Eventually he becomes the adulterous lover of Queen Guinevere, and it is this relationship that causes Arthur's Camelot to finally end. Other early prose mentioning Lancelot ois covered here.
The Vulgate Cycle follows chronologically after Chretien de Troyes work. The Vulgate Cycle is a comprehensive trilogy ('Lancelot Propre', 'La Queste del Saint Graal', and 'La Mort de Roi Artu'), believed to have been compiled by Cistercian monks between 1215 and 1235. It marks the transition between verse and prose in Arthurian legend.
Malory wrote his stories 300 years after Chretien de Troyes, by then Lancelot had become more popular than Gawain (Malory portrays Gwain as a knight bent on revenge). In Malory, Lancelot is Arthur's favorite, even athough he kills Gawain's brothers and commits adultery with Guinevere. For this sin, Lancelot can never see the Holy Grail. And the final tragedy is that eventually Arthur and Lancelot end up battling each other as Camelot tears itself apart.
Lancelot is loved by Elaine of Astolat (the daughter of King Pelles). Elaine tricks him - he thought she was Guinevere - into sleeping with her (and their union results in the birth of Galahad). Eventually his son Galahad is to see the Holy Grail and fulfill the Grail Quest; while Lancelot himself only saw a blurred vision and died knowing that his own sins had resulted in him to be unable to see the grail. Elaine meanwhile dies of grief as Lancelot does not return her love.
Lancelot has , of couse, been covered in literature down through history. Some modern prose and films are covered here.