The General Theory of Relativity paradoxically came second when Einstein expanded, i.e., generalized, the original Special Theory of Relativity in 1916. This exciting new General Theory deals with the curve of space and the effect on time of curved space. The theory's dominant idea explains the effect of gravity on the shape of space and the movement, or flow, of time. In essence, the gravity generated by matter causes space to curve toward the matter thus altering the flow of time, either speeding it up as it bends inward to the mass of matter or slowing it down as it curves around the outward curvature of space.

http://archive.ncsa.illinois.edu/Cyberia/NumRel/GenRelativity.html