Quantum mechanics is a fundamental theory in physics that describes the behavior of matter and energy at the smallest scales, such as atoms and subatomic particles. It introduces concepts like wave-particle duality, where particles can exhibit properties of both waves and particles, and superposition, where a particle can exist in multiple states at once until measured. This theory challenges our classical understanding of the universe, leading to phenomena that seem counterintuitive, such as entanglement, where particles become interconnected regardless of distance. Quantum mechanics has profound implications for technology, including the development of quantum computing and quantum cryptography.