Professor of Columbia University Brian Greene explains Superstring Theory, which is currently the most promising unifying theory, i.e. attempt to explain all four fundamental forces of nature. The four fundamental interactions are: strong nuclear force, weak nuclear force, electromagnetic force, and gravity. Brian is a prominent popularizer of science, and is best known for hosting and narrating PBS documentary The Elegant Universe, which won a Peabody Award in 2003.
Original video on TED.com (might be better quality).