Classical mechanics describes the motion of macroscopic objects such as spacecraft, planets, stars, and galaxies. The classical theories are simple, but this branch of mechanics cannot be applied to tiny particles moving at very high speed, as the results may turn inaccurate. Quantum Mechanics has much more complicated theories than classical mechanics (thanks to Einstein), but provides accurate results for particles of even very small sizes. The theory of relativity by Einstein (1905) deals with particles of tiny sizes while General theory of relativity by Einstein (1916) can be used to study all particles in general. Classical Mechanics is preferred to General theory of relativity for particles of macroscopic sizes, just because of its simplicity. Quantum mechanics is the branch of physics relating to the very small.
Quantum mechanics (QM) developed over many decades. Quantum mechanics is thus not deterministic, but probabilistic. Quantization helped to explain other mysteries of physics. In 1907, Einstein used Planck’s hypothesis of quantization to explain why the temperature of a solid changed by different amounts if you put the same amount of heat into the material but changed the starting temperature. In the 1920’s, physicists developed mathematical laws which describe quantum behavior. In particular, Erwin Schrodinger and Werner Heisenberg developed the key mathematical laws governing quantum particles ( Schrodinger’s Wave Equation and Heisenberg Matrix Mechanics).