Geologic periods are divisions of time that are used to describe the Earth's history. They are divided into four categories: EONS, ERAS, EPOCHS, and idyls. EONS are the longest divisions of time, lasting hundreds of millions of years. ERAS are shorter than EONS, lasting tens of millions of years. EPOCHS are even shorter, lasting millions of years. Idyls are the shortest divisions of time, lasting thousands of years. Together, these four categories provide a comprehensive timeline of the Earth's history.