General Relativity

General Relativity is basically a theory of gravity. It "relates" space and time, and uses that relation to define gravity as a fabric of space-time. Like in this picture, anything that has mass creates a curve or a depression in the fabric. The more mass an object has, the more deep the curve.

Similarly, in the solar system, the Sun creates a massive curve and attracts all planets towards it using a centripetal or center-seeking force. Thankfully, the planets are moving tangentially at great speeds too, and are under the effect of centrifugal or center-fleeing force. These balance and the planets remain at the same distance. 

This is it, gravity being a curve in the fabric of space-time, that's general relativity.