The Phanerozoic Age or Aeon is the geological time since life become abundantly visible in the fossil record, that is since the Cambrian Explosion starting about 570 million years ago.
Before the Phanerozoic Aeon was the Proterozoic Aeon, also known as the Precambrian, in which the dominant forms of life were unicellular. The difference in fossil records is because of the evolution of hard body parts, which got preserved more clearly.
The Phanerozoic is divided into three Eras, the Palaeozoic, the Mesozoic, and Cenozoic or Cainozoic, which goes up to the present. These are divided one from another by the great extinctions: the Permian extinction at the end of the Palaeozoic was the largest ever, and the second-largest was the K/T extinction at the end of the Cretaceous, the last period in the Mesozoic: the dinosaurs became extinct at the K/T boundary.
The name comes from the Greek for visible life.