Helen was the daughter of Leda and Tyndareus (though her true father was Zeus). She was reputed to be extremely beautiful, and when she reached the age to marry, many men, chiefs and kings came to court her. Realising the difficult situation the family were in, Tyndareus (via the inspiration of Odysseus) made all the suitors swear an oath - that they would come to the aid of whoever won Helen, should need arise. Menelaus was chosen (his brother, Agamemnon, being already married to Helen's sister, Clytemnestra), he and Helen married, and in time they had a daughter, Hermione.
Meanwhile, across the sea in what is now Turkey, a young shepherd/prince called Paris was chosen by the gods to be an arbitrator in a contest between three goddesses, as to who was the most beautiful. Each goddess offered him a bribe, Athena offered success in battle, Hera offered rule over Asia, and Aphrodite promised him the most beautiful woman in the world as his wife (i.e. Helen). Paris chose Aphrodite, and thus set in motion the start of the Trojan War.
Paris came to Sparta, the home of Helen and Menelaus, and visited as a royal guest. When Menelaus had to depart to Crete, he left Helen to look after their guest, at which point Paris seduced, or raped her (depending on which variant you read). Paris took Helen and many rich goods back with him to Troy. When Menelaus discovered the betrayal he called in the suitors' oath, and a great Greek army was amassed and sailed to Troy to reclaim Helen. The war lasted ten years, many heroes were killed, and the gods joined in on both sides. Finally the Greeks won, with a little trickery (i.e. the wooden horse) and Helen and Menelaus were reunited. They sailed back to Greece and lived out the rest of their lives together.
There are many variants on the basic story (and many other stories interconnected), and the issue of whether Helen chose to go with Paris or was raped has generated much discussion. Within ancient sources she is usually castigated for being a loose woman, blamed for all the deaths in the Trojan War, and almost universally hated. Her actions do not demonstrate her to be a particularly bad woman, sometimes a little selfish or misguided, but not the evil whore many other characters accuse her of being.
The story of a woman whose abduction caused a ten year war would not at first glance seem particularly illuminating for the story of a goddess or a heroine, but a closer look will reveal just how special Helen was.