General relativity is firstly the generalisation of the principles of special relativity to accelerated reference frames.
But it also brings a new conception of space and time, in which gravitation becomes a geometrical property of space-time (which has the structure of a Riemann space). In other words, the geometry of space-time depends on the distribution of masses, and, because of the equivalence of mass and energy, of energy flows.
The first experimental tests of the theory of general relativity were in the early 1920s (the curvature of light rays by a gravitational field was tested near to the Sun by observing the precession of the perihelion of Mercury), but the theory has now successfully passed many other tests, one of the most precise of which being the proof of the existence of gravitational waves (predicted by general relativity) through the minute observation of the evolution of a binary pulsar by Taylor and Hulse (Nobel prize 1993).
Among the most well known consequences of general relativity are the existence of black holes (cf. the work of Schwartzschild), the expansion of the universe etc., and the application of this theory to astrophysics and cosmology has turned out to be particularly fruitful.