Geologists' Periods are divided into two categories: ERAS and EONS. ERAS are the longest divisions of geological time, lasting hundreds of millions of years. They are further divided into smaller units such as periods, epochs, and ages. EONS are the largest divisions of geological time, lasting billions of years. They are further divided into ERAS, periods, epochs, and ages. Geologists use these divisions to study the Earth's history and the evolution of life on the planet.