Short answers, pulled from the story.
Helen of Troy, also called Helen of Sparta and in Latin Helena, was a figure in Greek mythology said to be the most beautiful woman in the world. She was first married to King Menelaus of Sparta, and her later marriage to Paris of Troy was the most immediate cause of the Trojan War.
In most sources, including the Iliad and the Odyssey, Helen was the daughter of Zeus and Leda, the wife of the Spartan king Tyndareus. In the Cypria she is instead the daughter of Zeus and the goddess Nemesis, and several writers describe her being born from an egg.
Most Ancient Greek sources, following Homer, held that Helen fell in love with Paris and went to Troy willingly. The scholar Ruby Blondell notes that none of the surviving sources claims Paris took Helen by force against her will, though some later versions depict her as forcibly abducted.
The Oath of Tyndareus was a solemn promise, proposed by Odysseus, in which all of Helen's suitors swore to defend her chosen husband against anyone who quarreled with him. When Paris took Helen, Menelaus called on the suitors to fulfill that oath, which began the Trojan War.
At least three Ancient Greek authors, Euripides, Stesichorus and Herodotus, denied that Helen ever went to Troy and said she spent the war in Egypt. In Euripides' version Hera made a likeness of Helen from clouds, and Hermes took the real woman to Egypt, where she waited in Memphis for ten years.
Helen's fate varies by source. Pausanias records that after Menelaus died, his sons drove her out and she fled to Rhodes, where Polyxo had handmaidens dressed as Furies hang her from a tree. In other versions she is taken up to Mount Olympus and made a sea goddess, or she simply returns to Sparta and lives with Menelaus.