This theory actually encompasses two distinct parts:

* Special Relativity: This theory, published in 1905, deals with the relationship between space and time, and how they are perceived by observers in different frames of reference. It introduced the famous equation E=mc², which states that energy and mass are equivalent.

* General Relativity: Published in 1915, this theory extended special relativity to include gravity. It describes gravity not as a force, but as a curvature of spacetime caused by the presence of mass and energy.

While Einstein is known for other significant contributions to physics, the theory of relativity is undoubtedly his most famous and influential work.