An eon is the largest division of geological time, spanning billions of years. The Earth's history is divided into four main eons: Hadean, Archean, Proterozoic, and Phanerozoic. Each eon is characterized by significant geological and biological events, such as the formation of the Earth, the emergence of life, and the development of complex organisms. Eons are further divided into smaller units called eras, which are then broken down into periods. The Phanerozoic eon, for example, includes three eras: Paleozoic, Mesozoic, and Cenozoic. Understanding eons helps scientists study the Earth's history and the evolution of life over time.