General relativity is a description of gravity in terms of spacetime geometry, characterizing acceleration due to gravitational force as constant motion within a curved spacetime. In other words, what I feel as the Earth's gravitational force is an artifact of the space I'm occupying moving in that direction relative to the Earth (and since I'm standing still on Earth, I'm moving "upward" relative to that space, and in fact, accelerating).
General relativity consists of the mathematical equations that provide such a characterization consistent with Newton's laws and special relativity. It relegates gravitational force to be an artifact, like centrifugal force or Coriolis force. It was developed by Einstein in 1916 following his earlier development of special relativity. General relativity remains the current favored model of gravity, based on its unique ability to explain the common, intuitive behavior of gravity as well as observed extreme cases that defy that "common" behavior. It is encapsulated in Einstein's field equation or Einstein's equation:
1 8πG Rμν - ——— gμνR + gμνΛ = ———— Tμν 2 c4
μ and ν each indicate the four dimensions of spacetime, i.e., Rμν, gμν and Tμν each indicate a relationship between the four individual scalar values (a tensor, e.g., a 4×4 matrix).
Of interest is Λ, the cosmological constant, which Einstein included to compensate for the formula's implication that the universe would otherwise be expanding or contracting. Upon Edwin Hubble's later discovery of the visible expansion of the universe, Einstein no longer saw reason to include it. More recent observations showing expansion not following general relativity's predictions has induced physicists to revive the term to make the equation to match observed reality, hypothesizing dark energy to explain it.