Defines inertial and gravitational mass, their scientific history, formulas used in their calculation, the definition of weight, units used for mass, and mass as perceived in the theory of special relativity.

This essay from the NOVA Web site explores the impact Einstein made on physics and most everything we know about the cosmos.

At this site from the Georgia State University is where the proof all starts. One must understand this experiment to have any confidence in relativity. Here's the experimental setup and a summary of the data.