Helen (often called "Helen of Troy") was the daughter of Leda and Zeus, and was the sister of the Dioscuri and Clytemnestra.
Since Zeus visited Leda in the form of a swan, Helen was often presented as being born from an egg. She was reputed to be the most beautiful woman in the world. When Helen was still a child, she was abducted by Theseus. Since she was not yet old enough to be married, he sent her to Aphidnae and left her in the care of his mother, Aethra. The Dioscuri rescued her and returned her to her home in Lacedaemon, taking Aethra prisoner at the same time.
When Helen reached marriageable age, all the greatest men in Greece courted her. Her mother's husband, King Tyndareos of Lacedaemon, was concerned about the trouble that might be caused by the disappointed suitors. Acting on the advice of Odysseus, he got all the suitors to swear that they would support the marriage rights of the successful candidate. He then settled on Menelaus to be the husband of Helen. She lived happily with Menelaus for a number of years, and bore him a daughter, Hermione.
After a decade or so of married life, Helen was abducted by -- or ran off with -- Paris, the son of King Priam of Troy. Menelaus called on the other suitors to fulfill their oaths and help him get her back. As a result, the Greek leaders mustered the greatest army of the time, placed it under the command of Agamemnon, and set off to wage what became known as the Trojan War.
After the fall of Troy, Menelaus took Helen back to Lacedaemon, where they lived an apparently happy married life once more. After the end of their mortal existence, they continued to be together in Elysium.
There were a number of different accounts of Helen's relationship with Paris. In some, she was truly in love with him, although her sympathies were mostly with the Greeks who beseiged Troy. In others, she was a beautiful and wanton woman who brought disaster upon those around her. In still other accounts, she never went to Troy at all: Hermes, acting on Zeus's orders, spirited her away to Egypt and fashioned a phantom out of clouds to accompany Paris; the real Helen was reunited with Menelaus after the Trojan War.