Information from: http://www.ramsdale.org/legend.htm#G...

Arthur was the illegitimate son of Uther Pendragon, king of Britain, and Igraine, the wife of Gorlois of Cornwall.

After the death of Uther, Arthur, who had been reared in secrecy, won acknowledgment as king of Britain by successfully withdrawing a sword from a stone.

Morgan le Fay was usually represented as an evil sorceress, scheming to win Arthur's throne for herself and her lover. Mordred (or Modred) was variously Arthur's nephew or his son by his sister Morgawse. He seized Arthur's throne during the king's absence. Later he was slain in battle by Arthur, but not before he had fatally wounded the king. Arthur was borne away to the isle of Avalon, where it was expected that he would be healed of his wounds and that he would someday return to his people.

Merlin: magician, seer, and teacher at the court of King Vortigern and later at the court of King Arthur. He was a bard and culture hero in early Celtic folklore. In Arthurian legend he is famous as a magician and as the counselor of King Arthur. In Tennyson's Idylls of the King Merlin is imprisoned eternally in an old oak tree by the treacherous Vivien (or Nimue), when he reveals the secrets of his knowledge to her.
Lady of the Lake: a misty, supernatural figure endowed with magic powers, who gave the sword Excalibur to King Arthur. She inhabited a castle in an underwater kingdom. According to one legend she kidnapped the infant Lancelot and brought him to her castle where he lived until manhood. She has been identified variously with Morgan le Fay and Vivien. The poem The Lady of the Lake, by Sir Walter Scott, is based on a totally different legend.

Avalon: in Celtic mythology, the blissful otherworld of the dead. In medieval romance it was the island to which the mortally wounded King Arthur was taken, and from which it was expected he would someday return. Avalon is often identified with Glastonbury in Somerset, England.

Guinevere: wife of King Arthur. Her illicit and tragic love for Sir Lancelot, which foreshadowed the downfall of Arthur's kingdom, ends with her retirement to a convent. She also figures in several early romances and Celtic legends, her name appearing in various forms (e.g., Guanhamara, Gvenour, and Gwenhwyfars). In different versions of the Arthurian story her name appears as Guenevere and Guinever.

Sir Lancelot: bravest and most celebrated knight at the court of King Arthur. He was kidnapped as an infant by the mysterious Lady of the Lake, from whom he received his education and took his title, Lancelot of the Lake. As a young man he went to the court of King Arthur, where he was knighted and became one of the most feared warriors in all Christendom. Lancelot was the lover of Guinevere, his sovereign's queen. He was also loved by Elaine (the daughter of King Pelles), by whom he was the father of Sir Galahad, and by Elaine, the Lily Maid of Astolat, who died for love of him. Lancelot’s name sometimes appears as Lancelot.