Thor at the birthday party yesterday...

Special theory of relativity:
It explains why the speed of light is always the same, no matter what speed an observer is travelling. It predicts that observers travelling at different speeds relative to each other will witness the same event happening in a different way (space and time become altered). It gives the formula for time dilation as one approaches the speed of light. Space and time were once thought to be absolute, but Einstein showed them to be relative depending on the speed of an observer (speed is the measure of space-time like distance is the measure of space).

General theory of relativity.
Explains that acelleration is the same force as gravity. Acelleration traces a curve through space-time, while gravity is a curvature of space-time itself (space-time being a concept invented by Einstein in the special theory). Any object that enters a gravitational field therefore undergoes acelleration, similar to how a "straight" line drawn on a curved surface follows that curve.