The Theory of Relativity was developed by Dr Albert Einstein, and comes in 2 parts. The first is the Special Theory of Relativity, which was published in 1905 (or is it 1906?). In it, the speed of light was taken as a constant (an assumption from the results of Maxwell's Equations). From there, the time dilation factor was developed. The relativistic equation for kinetic energy was developed, and so was the idea of mass/energy conservation. The relativistic equation for the doppler shift for light was also developed. From here, substituting the Newtinian equation for kinetic energy E=(1/2)mv^2 (for speeds far below the speed of light, relavant to the speed of atoms), the equation e=mc^2 was developed. The General Theory of Relativity was published in 1911. i know nuts about it.
It's called the theory of special/general relativity because it makes a couple of assertions about how things relate to one another. First, it states that when things move you observe them as moving (or not moving) relative to yourself. So, when you drive down the road with a doughnut in your hand, you (and the doughnut) are moving at 100 km/h. However, when you go to bite the doughtnut, you're not worried about it suddenly smacking you in the face at 100 km/h - relative to you, it is not moving. The difference of relative motion is important because it actually changes the way that time flows for you with respect to other things. This is what Arch_Rival was talking about with Special Relativity, which Einstein originally came upon (so I've been told) when he was trying to explain the precession of the planet Mercury. (That is, why it seemed to be in a different place from where Newtonian physics predicted it should be.) I'm on shaky ground here 'cause I'm not a physicist, and someone may soon correct me. The important thing about general relativity is that the speed of light is always constant for all observers even though this is not true for the speed of other objects, which leads to some situations that seem paradoxical for most people.
