The classification of finite simple groups is a major achievement in the field of group theory, a branch of mathematics. Finite simple groups are the building blocks of all finite groups, similar to how prime numbers are the building blocks of integers. The classification theorem states that every finite simple group belongs to one of several categories, including cyclic groups, alternating groups, and Lie groups. The proof of this classification is extensive and complex, spanning thousands of pages and involving contributions from many mathematicians. It was completed in the late 20th century and is often referred to as the "enormous theorem" due to its size and depth. Understanding these groups helps mathematicians analyze the structure and properties of more complex groups.