The Illiad is a Greek epic attributed to Homer about the siege of Troy.
Not unlike the much larger Indian epic, the Mahabharata, this Greek tale involves a grand-scale war between two opposing forces. Divine gods and goddesses often appear and, like the Indian story, offer their assistance to favored mortals.
Together with the Odyssey, the Illiad is one of the pillars of existing Greek myth.
To be honest, I found the Odyssey more personally engaging. So rather than my trying to summarize the story, it is better to leave that to an expert. For an excellent outline with commentary and original Greek terms, see Sir Paul Harvey’s work, freely available at http://www.archive.org/details/oxfordcompaniont006050mbp (PDF, page 220).
Related » Aeneas, Aesculapius, Athena, Hesiod