Eras is a term used by geologists to describe the divisions of geological time. It is divided into two major categories: the Phanerozoic Eon, which includes the Paleozoic, Mesozoic, and Cenozoic ERAS, and the Precambrian Eon, which includes the Hadean, Archean, and Proterozoic ERAS. Each of these ERAS is further divided into periods, epochs, and ages. Geologists use these divisions to study the Earth's history and the evolution of life on the planet.