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It was invented by Albert Einstein in 1916, who expanded his Special Theory to include gravity's effect on spacetime. The Special Theory, published in 1905, stated that the speed of light is the same no matter where or how you're observing it, and that observers moving at a constant speed should encounter identical physical laws.
The theory of general relativity has been backed up by several observations. In 1919, during a solar eclipse, scientists discovered that light was bent around the sun (known as gravitational lensing) to the exact degree of Einstein's theory; general relativity also accounted for the small changes in Mercury's elliptical orbit, which had gone unexplained by Isaac Newton's theory of gravity. Furthermore, gravitational redshift (where photons--light--lose their energy and move toward the red end of the spectrum) has also been observed.
Gravity waves (waves or "ripples" in spacetime) predicted by the theory have not been recorded to date, though the Laser Interferometer Gravitational Wave Observatory (LIGO) is currently looking for them.
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Notable scientist who has studied general relativity:
Michio Kaku
Michio Kaku is a theoretical physicist who graduated from the University of California, Berkeley, and currently holds the Henry Semat Chair and Professorship in theoretical physics. He teaches at the City College of New York and has been a visiting professor at New York University and the Institute for Advanced Study, Princeton. He is the host of the two shows Visions of the Future and The Universe, along with the host of two radio programs Science Fantastic and Explorations in Science. His books include Beyond Einstein, Visions, Einstein's Cosmos, Parallel Worlds, and Physics of the Impossible: A Scientific Exploration of the World of Phasers, Force Fields, Teleportation, and Time Travel.
His current work involves unifying the four fundamental forces--electromagnetism, strong force, weak force, and gravitation--and string theory, which he co-founded.
Video of Michio Kaku:
Watch Michio Kaku: Theoretical Physicist on PBS. See more from Secret Life of Scientists.
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Sources:
http://archive.ncsa.illinois.edu/Cyberia/NumRel/GenRelativity.html
http://archive.ncsa.illinois.edu/Cyberia/NumRel/SpecialRel.html
http://astro.physics.sc.edu/selfpacedunits/Unit57.html
http://www.astro.ucla.edu/~wright/bh-st.html
http://www.astro.ucla.edu/~wright/relatvty.htm
http://www1.ccny.cuny.edu/prospective/science/profiles/Kaku-Profile.cfm
http://www-history.mcs.st-and.ac.uk/HistTopics/General_relativity.html
http://mkaku.org/home/?page_id=5
http://www4.ncsu.edu/unity/lockers/users/f/felder/public/kenny/papers/gr1.html
http://www.pbs.org/wgbh/nova/physics/relativity-and-the-cosmos.html
http://simplycharly.com/einstein/michio_kaku_interview.htm
http://scienceworld.wolfram.com/biography/Einstein.html
http://scienceworld.wolfram.com/physics/GeneralRelativity.html
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Do you think string theory is the answer to the lack of unity between general relativity and quantum theory?
-----The Golden Eagle
