Quantum theories are scientific frameworks that describe the behavior of matter and energy at the smallest scales, such as atoms and subatomic particles. These theories challenge classical physics by introducing concepts like wave-particle duality, where particles like electrons can exhibit properties of both waves and particles. One of the most well-known quantum theories is quantum mechanics, which provides mathematical tools to predict the probabilities of different outcomes in experiments. Another important concept is quantum entanglement, where particles become interconnected in such a way that the state of one instantly influences the state of another, regardless of distance.