What is gravity? Newton left that question unanswered when he formulated his inverse square law of the gravitational force, framing no hypothesis for what agency transmits gravity, only asserting it was an action at a distance. Einstein brought gravity into relativity by answering Newton's unanswered question with his General Relativity, our modern theory of gravity. In Einstein's formulation, Matter tells spacetime how to curve, and curved spacetime tells matter how to move. This lecture presents the basic picture of General Relativity, and introduces some of its observational consequences. The surprising conclusion is that instead of space and time being a backdrop for physics in Newton's view, united into spacetime by Relativity they are understood to be physical and dynamic. This is important for understanding how the Universe as a whole works. Recorded 2006 February 21 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.