The Postulates of Special Relativity and General Relativity.

Einstein's Theory of Relativity has two main parts: Special Relativity and General Relativity. 

Special Relativity (1905):

1. Principle of Relativity: The laws of physics are the same for all observers in uniform motion relative to each other (i.e., in inertial frames of reference). There is no preferred frame of reference. 

2. Constancy of the Speed of Light: The speed of light in a vacuum is constant and is the same for all observers, regardless of their relative motion or the motion of the light source. 

General Relativity (1915):

1. Principle of Equivalence: Local observations in a freely falling reference frame (where gravity is negligible) are indistinguishable from those in an inertial frame (i.e., there is no difference between being at rest in a gravitational field and accelerating in space). 

2. Curvature of Spacetime: The presence of mass and energy curves spacetime, and this curvature affects the motion of objects, which we perceive as gravity. 

In results, Special Relativity deals with the relationship between space and time in the absence of gravity, while General Relativity extends these concepts to include gravity as a curvature of spacetime.