The Great Unconformity is arguably the most interesting single sight at the Grand Canyon. In geologic terms, an unconformity is a place in which there was an interruption in the depostion in sediment. During such interruptions, existing sediment can be eroded, folded, tilted, or otherwise messed with before new sediment is deposited on top of it. The Great Unconformity separates rocks of Precambrian age, identifiable because they are tilted, from later flat-lying Paleozoic rocks. The Precambrian rocks must have been tilted and before the Paleozoic rocks were deposited, or else those would be tilted, too.
There are few places in the world with public access that so clearly show the reality of geologic time. Even if we had no idea that the rocks we are looking at were Precambrian or Paleozoic, it would be clear that much time large amounts of time passed between the depostion of the Precambrian and Paleozoic rocks - enough time for tilting and erosion.
The Precambrian rocks belong to the Grand Canyon Group. This group is only visible in the eastern end of the canyon. This photo was taken from the Desert Overlook, at the southeast rim.