Quantum theory is a fundamental branch of physics that describes the behavior of matter and energy at very small scales, such as atoms and subatomic particles. It introduces concepts like wave-particle duality, which suggests that particles, like electrons, can exhibit properties of both waves and particles. This theory helps explain phenomena that classical physics cannot, such as the behavior of electrons in an atom. One of the key principles of quantum theory is superposition, which states that particles can exist in multiple states at once until measured. Another important concept is entanglement, where particles become interconnected in such a way that the state of one instantly influences the state of another, regardless of the distance between them.