Geologic times are divided into EONS and ERAS. An eon is the largest division of geologic time and is typically hundreds of millions of years long. ERAS are smaller divisions of geologic time and are typically tens of millions of years long. EONS and ERAS are further divided into periods, epochs, and ages. Together, these divisions of geologic time help us understand the history of the Earth and its life forms. The term 'geologic times' refers to the entire span of Earth's history, from its formation to the present day.