In this video, Minute Physics does a really nice job of explaining the missing part of the most famous equation in the world, E = mc2. It seems that that simple version of the equation only holds true for objects that are not moving. These moving objects have momentum which is represented as p in the slightly more complex equation, E²=(mc²)²+(pc)². If you pay attention, you'll even see why it is that, according to Einstein, nothing with mass can move at the speed of light (or faster).
Watch and enjoy.