The Lady of the Lake is the woman who came out of the Lake of Avalon and gave Arthur his magical sword, Excalibur.
She is the one who foster mothered Lancelot when he was a child, and cured him when he went mad.

Her real name is thought to be Vivien, and she is compared to many different legendary characters. She has characteristics of the sea nymph Thetis of Greek mythology. She also has a lot in common with Morgan le Fey, the powerful sorceress, occasional antagonist of King Arthur and Guinevere, and daughter of Lady Igraine.
Vivien learns her magic from Merlin, who was madly in love with her. She started out as his scribe but eventually became so powerful that she eventually used her magic against him and trapped him for eternity in a dungeon of some sorts (different sources say it was stone, a tree, or a cave).
She sometimes helped guide Arthur but by trapping Merlin, she basically caused his downfall. She reclaimed her sword when Arthur was fatally wounded at the Battle of Camlann. She was also one of the three queens who took the King to the island of Avalon.
Lady of the Lake
her celtic name Argante means silver one. This is a painting by Launcelot speed in 1919.
Several women, who are often blurred in the legends, all carry this title. Some names include Viviane, Eviene, Viviene, Nimue, and Nina. In Malory she is the woman who gives Arthur Excalibur and later takes it back when it is thrown into the water. In Malory, she is killed in Arthur's court by Balin, but later reappears, which leads to the speculation that the Lady of the Lake is a sort of title. Marion Zimmer Bradley uses this idea to its full advantage in her feminist revision of the tales, The Mists of Avalon. The Lady of the Lake plays a crucial role in this text.

water deities were popular amongst Celtic Society because the controlled the essence of life itself. springs, rivers and lakes show the supernatural powers of the goddesses who lived within. offerings were common, such as weapons and other valuables. this practice is still alive and contiues at wishing wells across the country. now the lady of the lake is known as "lady luck".(3)
http://www.kingarthursknights.com/others/ladylake.asphttp://www.britannia.com/history/biographies/nimue.html(3)http://www.britannia.com/history/biographies/nimue.html