The motion of a cannonball, the orbit of a planet, the working of an engine, the motion of a baseball bat, ... are described by classical mechanics.
Quantum mechanics explains the behaviour of matter and its interactions with energy on the scale of atoms and subatomic particles.
Classical mechanics is wrong in principle, but it is a good approximation to
relativistic mechanics when applied to objects moving at low speeds.
In 1905, Albert Einstein realized that these ideas didn't apply to objects moving at high speeds (~ the speed of light) and he developed an alternative body of ideas called relativistic mechanics.
