relative principle: the laws of physics are the SAME in all inertial reference frames and general theory: basically is einstiens tackled problem of accelerating referencing frames and developed a theory of gravity.
I got this outta Physics, 5th edition by GIANCOLI.
Good Luck, I hope this helps.
Dx
The special Theory of Relativity has 2 postulates.
1. All physical laws are the same in ineterial frames of reference.
2. The speed of light is constant.
From these postulates Einstein derived the Lorentz transforms. These in turn led to c as an universal upper speed limit.
I will let others give a 25 word or less explanition of GR.