General relativity (GR) is the geometrical theory of gravitation published by Albert Einstein in 1916.
It unifies Einstein's earlier special relativity with Sir Isaac Newton's law of universal gravitation.
This is done with the insight that gravitation is not due to a force but rather is a manifestation of curved space and time.
In general relativity, spacetime is non-Euclidean, or curved.
The need for curvature arises from the equivalence principle and a child's simple question: "What keeps the people on the other side of the world from falling off?".
In other words, should not the inertial paths on the other side of the Earth take objects away from the planet?
Instead, all free-fall trajectories in the vicinity of a massive object will draw objects towards it.