What is often referred to as "the theory of relativity" is actually two theories, general and special, that describe the way the universe works on its largest scale and at its highest energies. The three main premises of the theories are that matter and energy are interchangeable, space and time are difficult to distinguish in calculations and no frame of reference is better for viewing the universe than any other.
The interchangeability of matter and energy is the source of Albert Einstein's best-known equation, E=mc². This equation describes the amount of energy that can be liberated by converting matter as well as the amount of energy required to generate matter of a given mass.
The unification of space and time greatly simplified the equations used in classical physics and explained several apparent paradoxes. James Clerk Maxwell observed that a moving magnetic field induces an electric current in copper wires, but the effect is independent of whether the magnet is moving past the wires or the wires are moving past the magnet. In a break with classical physics, general relativity posits that no frame of reference is intrinsically superior to any other.
Special relativity deals with the way light behaves in a vacuum. The speed of light is a constant, and nothing with any mass can ever go faster. Two cars approaching each other at 50 mph each have a closing speed of 100 mph. Because of relativistic effects, however, two objects approaching each other at the speed of light have a closing speed of the speed of light, and no more.