Geologic time division is a way of categorizing the Earth's history into distinct units. It is divided into four main categories: EON, AGES, ERA, and Neocene. An EON is the largest unit of geologic time, spanning hundreds of millions of years. AGES are smaller divisions within an EON, usually lasting tens of millions of years. An ERA is even smaller than an Age, usually lasting millions of years. The Neocene is the smallest unit of geologic time, lasting thousands of years.