Short answers, pulled from the story.
The theory of relativity comprises two physics theories by Albert Einstein: special relativity and general relativity. Special relativity applies to all physical phenomena in the absence of gravity, while general relativity explains gravitation and its relation to the forces of nature.
Einstein published special relativity in 1905 and general relativity in 1915. He developed general relativity between 1907 and 1915, and its final form was published in 1916.
Special relativity rests on two postulates. The laws of physics are the same for all observers in any inertial frame of reference relative to one another, and the speed of light in vacuum is the same for all observers regardless of their relative motion or the motion of the light source.
Three experiments conducted between 1881 and 1938 were critical to validating special relativity: the Michelson-Morley experiment, the Kennedy-Thorndike experiment, and the Ives-Stilwell experiment. The Kennedy-Thorndike experiment was first performed in 1932, and the Ives-Stilwell experiment was carried out first in 1938 and with better accuracy in 1941.
Global positioning systems such as GPS, GLONASS, and Galileo must account for all relativistic effects to work with precision, including the consequences of the Earth's gravitational field. Each satellite moves relative to an Earth-bound user and is therefore in a different frame of reference under the theory.
The equivalence principle holds that accelerated motion and being at rest in a gravitational field, such as standing on the surface of the Earth, are physically identical. It led Einstein to conclude that free fall is inertial motion and that spacetime is curved.