Albert Einstein's theory of relativity consists of two parts: special relativity and general relativity. Special relativity, introduced in 1905, explains how time and space are interconnected, showing that time can slow down or speed up depending on an object's speed. It also established that the speed of light in a vacuum is constant for all observers, regardless of their motion. General relativity, published in 1915, expands on this by describing gravity as the curvature of space and time caused by mass. This means that massive objects like Earth and the Sun warp the space around them, affecting the movement of other objects and light.