Theories of relativity, developed by Albert Einstein, consist of two main parts: special relativity and general relativity. Special relativity, introduced in 1905, focuses on the physics of objects moving at constant speeds, particularly at speeds close to that of light. It introduces concepts like time dilation and length contraction, showing that time and space are interconnected. General relativity, published in 1915, expands on this by describing gravity as the curvature of spacetime caused by mass. This theory explains how massive objects, like planets and stars, influence the motion of other objects, fundamentally changing our understanding of gravity and the universe.