The Koppen climate classification system is the most widely used system for classifying the world's climates. It categorizes climates into five main groups - A, B, C, D, and E - based on annual and monthly averages of temperature and precipitation. Each group is further divided into more specific climate classifications. For example, the tropical climates under group A are divided into rainforest, monsoon, and savanna climates. The Koppen system provides a standardized way to quantitatively compare and characterize climates around the world.