The Meteor (also referred to as the Harbinger of Doom) is an unknown object that fell from the skies and landed on Tristram Cathedral, twenty years after the destruction of the Worldstone. It crashed with such force that it penetrated the depths of the Catacombs that are located under the Cathedral. It is said that this object awakened ancient evils, and called for the heroes of Sanctuary to rise against the powers of the Burning Hells once again.
Through the Diablo III Beta, Blizzard gave an official name to the meteor, naming it "The Fallen Star" as evident in many character quotes and as a quest name. What it actually is, however, is debatable. Brother Malachi the Healer refers to it as a comet in his prophecy regarding King Leoric, the Skeleton King.
As shown during the in-game cutscenes and storyline, the Meteor does indeed crash into the Tristram Cathedral. During Act 1 of Diablo III, the main focus is to get to the Meteor and discover what it is. It turns out that the Meteor is actually a dark-skinned man, who is later revealed to be the Archangel of Justice, Tyrael.