Special: light travels at a constant speed for all observers, plus the laws of motion are the same in all inertial (non-accelerating) frames. Makes all physical laws "covariant" (really invariant) under a certain group of coordinate transformations. Encodes quantities like energy and momentum with scalars and 4-vectors. Models spacetime as a manifold that is locally $\RR^3 \cross \RR$ with metric signature $(+,-,-,-)$. Works for reformulating classical laws, but doesn't incorporate gravity.

General: Want a theory that works for accelerating frames and incorporates gravity. Requires bringing together energy, momentum, stress, and pressure, and moving to $4\times4$ tensors instead of 4-vectors. Recover SR by defining inertial frames to travel along geodesics with constant velocity (setting acceleration equal to zero).