There are many stories about Queen Guinevere, written over a long period of time. The earliest ones probably came out of Wales, in Britain. In them, she has the name Gwenhwyfar. The most famous ones were written by a man named Sir Thomas Malory, who is thought to have been an English knight himself. His book was called "Le Morte d'Arthur," which means "The Death of Arthur." It was finished about 1485 and most of the modern ideas about who Guinevere was and what she did come from it.
According to Sir Thomas, Guinevere was the daughter of King Leodegrance in a place called Camelard -- not the same as Camelot. When her father was in trouble fighting another king, King Arthur brought 20,000 knights and rescued him. Afterwards there was a big celebration and it's there that Arthur first saw Guinevere and fell immediately in love with her. When they got married, she brought the Round Table with her from her father's house, along with 100 knights.
Queen Guinevere gets kidnapped a lot, usually by a wicked lord who either wants her for her beauty or wants to take Arthur's place. The name of some of her kidnappers in different stories include King Melwas of Somerset, King Arthur's nephew Mordred, Meleagant and Sir Meliagrance. Sometimes Arthur rescues her and sometimes Sir Lancelot rescues her. In one story, Lancelot comes after her in a cart driven by a dwarf, and has to crawl over a bridge whose top edge is as narrow and sharp as the blade of a sword.
In a story that was written in France, Queen Guinevere had an identical twin sister who persuaded King Arthur that she was the real queen and took her place. It took two and half years before he discovered the truth and was reunited with the real Guinevere. In another story, her sister is named Gwynhwyfach, and is the wife of Mordred, Arthur's nephew and enemy. They fought so much they ended up causing their husbands to go to war in the Battle of Camlan.