The Iliad (sometimes referred to as the Song of Ilion or Song of Ilium) is an epic poem in dactylic hexameters, traditionally attributed to Homer. Set during the Trojan War, the ten-year siege of the city of Troy (Ilium) by a coalition of Greek states, it tells of the battles and events during the weeks of a quarrel between King Agamemnon and the warrior Achilles. Although the story covers only a few weeks in the final year of the war, the Iliad mentions or alludes to many of the Greek legends about the siege, the earlier events, such as the gathering of warriors for the siege, the cause of the war and similar, tending to appear near the beginning, and the events prophesied for the future, such as Achilles' looming death and the sack of Troy, prefigured and alluded to more and more vividly approaching the end of the poem, making the poem tell a more or less complete tale of the Trojan War.
Along with the Odyssey, also attributed to Homer, the Iliad is among the oldest extant works of Western literature, and its written version is usually dated to around the eighth century BC. The Iliad contains over 15,000 lines, and is written in Homeric Greek, a literary amalgam of Ionic Greek with other dialects.
Source: Wikipedia, "Iliad", available under the CC-BY-SA License.