The Odyssey and The Iliad are two long, ancient poems first spread by being told aloud and passed down through generations by word of mouth. They were likely composed by the same poet, in the same format, with some overlapping characters.
The Poet
The Iliad and The Odyssey are attributed to Homer, a traveling Greek poet who was probably blind. However, there is academic speculation as to who Homer was, when he lived, and if he was, in fact, just one person. "Homer" might actually have been a generic name for any traveling storyteller and not an individual man. No conclusive evidence exists to prove this "Homeric question" because no written records were kept.
The Epic Poem
The Iliad and The Odyssey are the defining works in the genre of epic poetry and the oldest stories passed down from ancient Greece. Epic poems are characterized by their extensive length, oral format and use of heroic characters. Much of what we know of the values and religion of ancient Greeks comes from these works.
The Iliad
The Iliad is a narrative of the Trojan War, including its causes and legendary battles, with an emphasis on the hero, Achilles. It is the tragic and bloody story of the fall of Troy to the Greek armies after the Trojan prince Paris kidnapped the most beautiful woman in the world, a married Greek woman named Helen.
The Odyssey
The Odyssey, a bit more fantastical, tells of the trials of one of the Iliad's minor characters, Odysseus, as he travels home from the Trojan War to reunite with his wife and son. The trip takes 10 years because Odysseus, at the whim of the gods, must battle a cyclops, raging seas, secret loves and several monsters.
The Gods and Goddesses
Both works include frequent interference by the Olympian gods. Zeus, Hera, Athena, Apollo and other members of the Greek pantheon are portrayed as cruel, manipulative deities who toy with mortal men, even to the death, for their own pleasure. Humans in The Iliad and The Odyssey have very little free will and must live out their predetermined destinies.