Group theory is a branch of mathematics that studies the algebraic structures known as groups. Groups are sets equipped with a binary operation that satisfies certain properties, such as closure, associativity, identity element, and invertibility. In group theory, mathematicians study the properties of groups, including their subgroups, homomorphisms, and group actions. Group theory has applications in various areas of mathematics, such as geometry, number theory, and cryptography. One of the key concepts in group theory is the notion of symmetry, as groups often arise as symmetries of geometric objects or algebraic structures. Group theory also plays a crucial role in the classification of finite simple groups, a significant achievement in modern mathematics. Overall, group theory is a foundational subject in mathematics with a wide range of applications and connections to other areas of mathematics.