The theory of relativity has two parts - the special theory of relativity and the general theory of relativity.
The former concerns the laws of physics in non-accelerating frames of reference. It postulates the constancy of the speed of light and the equivalence of the laws of physics in any inertial frame of reference.
The latter explains gravity as the curvature of spacetime caused by the presence of massive objects, which is more comprehensive than Newton's law of universal gravitation.
The theory of relativity has revolutionized our understanding of space, time, and gravity and is pivotal in modern physics.