It is about the Trojan war. The name comes from Illion, the ancient greek name of Troy. The legend says Paris, the trojan king's son, steals Helen, the beautiful wife of Menelaus, Sparta's king [sparta being a city-state in ancient Greece]. So the greeks start a war against the trojans to get Helen back. The war lasts for 10 years, until the greeks build a giant wooden horse and the army hides inside of it. They put the horse in front of Troy's gates claiming it is a gift in order to make peace between the 2 armies and to end the war. The trojans believe it, so they bring the horse inside the city and then they start to drink and party. At night, when everbosy was drunk or asleep, the greeks come out of the wooden horse and take the trojans by surprise. And so they conquer and destroy the city of Troy.
It's a good play, read it for yourself. Also, it's spelled "Iliad."
You should read it, it's a good one.