Briefly, The Iliad is about the Trojan War, a conflict that began when Helen, the queen of Sparta, fell in love and ran away with Paris, a prince of Troy. Helen's husband, Menelaus, was a the brother of Agamemnon, the powerful king of Mycenae. As high king over all the kings of other city-states, Agamemnon was able to muster a huge navy to sail to Troy to avenge his brother's humiliation--and gain a rich seaport if his forces won. Troy was a heavily fortified city and had never been defeated. The Trojan people survived a ten-year seige that ended only when the were tricked by the gift of the Trojan Horse, inside which Greek soldiers were hiding. When the people were drunk from celebrating the apparent retreat of the Greeks, the men who had been hiding climbed out of the horse and opened the gates, allowing the full Greek forces inside to raze the city and kill its inhabitants.
It's a powerful story that still entertains us today.