Quantum theory is a branch of physics that aims to explain the behavior of matter and energy at the smallest scales of atomic and subatomic particles. It revolutionized the field of physics in the early 20th century, introducing the concept of wave-particle duality and the uncertainty principle. Quantum theory has led to the development of technologies such as quantum computing, cryptography, and teleportation, and continues to be a focus of research in understanding the fundamental nature of the universe.