(Half) a Lecture on D-branes

This is a concise foreword to, rather than a review of D-brane physics.Comment: 20 pages, Latex. To appear in Proceedings of the Workshop on Gauge Theories, Applied Supersymmetry and Quantum Gravity (Imperial College, London, July 1996). Two misprints in eqs. (3.24) and (3.25) correcte