The laws of physics are the same for observers in all inertial frames of reference.

The Principle of Relativity is easy to understand once you understand that an inertial frame frame of reference is any point of view that is not accelerating. The principle states that whether I'm moving 20 miles per hour or standing still, the laws of physics must be the same for me. One consequence of the principle of relativity is that it is impossible to say whether or not something is moving except when comparing it to something else. The Principle of Relativity has been around for ages. It is sometimes even referred to as the Galilean Principle of Relativity.

The speed of light in vacuum has the same value c in all directions and in all inertial frames of reference.

In simple and concise terms, the postulate of special relativity was a natural result of Maxwell's Equations. When Maxwell formulated his famous set of equations not only did he succeed in describing the laws governing the phenomenon of electromagnetism, but he also left a very serious problem. The problem was inherent in his equations and their use of the permeability and permittivity of free space which are both related to the speed of light. This was all fine for normal near-inertial applications, but the use of these numbers presented problems if an object were accelerated. The laws would behave differently in an accelerated reference frame. This implied, it seemed, one of two different things, either Maxwell's Equations were fundamentally flawed because they disobeyed the principle of relativity, or the principle of relativity was incorrect because there existed a real law which acted against the principle.

The assumption was that the speed of light, like the speed of most observable objects, was variable depending upon how one moved relative to it. In short that the speed of light was relative. One odd, and possibly amusing fact about Einsteins special relativity is that, in fact, it is based upon the postulate that light's speed is actually not relative, but constant.

The idea that light appears to move at the same speed no matter how fast one was moving or in what direction, was an idea that required major modification to the laws of motion and of course Einstein spent a great deal of time making the necessary modifications and showing that they could work. The results implied that space and time were not so solid and inflexible as we might like to think.