What is Relativity all about (Briefly)?

Well one of the focus of relativity has to do with measurements of events (things that happen): where and when they happen, and by how much any two events are separated in space and in time. In addition, relativity has to do with transforming such measurements and others between reference frames that move relative to each other. Hence the name relativity.

Transformations and moving reference frames were well understood and quite routine to physicists in 1905. Then Albert Einstein published his groundbreaking Special theory of relativity. The adjective special means that the theory deals only with inertial frame of reference, which are frames in which Newton's laws are valid. This means that the frames do not accelerate instead, they can move only at constant velocities relative to one another. Then comes Einstein's General theory of relativity, it treats the more challenging situation in which frame of reference accelerate.

Starting with two deceivingly simple postulates, Einstein stunned the scientific world by showing that the old ideas about relativity were wrong, even though everyone was so accustomed to them that they seemed to be unquestionable common sense. This supposed common sense, however, was derived from experience only with things that move rather slowly. Einstein's theory of relativity, which turns out to be correct for all possible speeds, predicted many effects that were, at first study, bizarre because no one has experienced them.

In particular, Einstein demonstrated that space and time are entangled; that is, the time between two events depends on how far apart they occur, and vice versa. Also, the entanglement is different for observers who move relative to each other. One result is that time does not pass at a fixed rate, as if it were ticked off with mechanical regularity on some master grandfather clock that controls the universe. Rather, the rate is adjustable: Relative motion can change the rate at which time passes, Prior to 1905, no one but few daydreamers would have thought that. Now, engineers and scientists take it for granted because their experience with special theory of relativity has reshaped their common sense.

Special theory of relativity has the reputation of being difficult. It is not difficult mathematically, at least not here. However, it is difficult in that we must be very careful about who measures what about an event and just how that measurement is made and it can be difficult because it can contradict experience.  

The two postulates of relativity, 

1. The Relativity Postulate: The laws of physics are the same for observers in all inertial frame of reference. No frame is preferred.
   
   Galileo assumed that the law of mechanics was the same in all inertial frames of reference. Newton's first law of motion is one important consequence, Einstein extended that idea to include all the laws of physics, especially electromagnetism and optics. This postulate does not say that the measured values of all physical quantities are the same for all inertial observers; most are not the same. It is the laws of physics, which relate these measurements to each other, that are the same.
   
   
2. The Speed of Light Postulate: The speed of light in vacuum has the same value c which is 299,792,458 m/s in all directions and in all inertial frame of reference. 
   
   We can also phrase this postulate to say that there is in nature an ultimate speed c same in all directions and all frame of reference. Light happens to travel at this ultimate speed, as do any massless particles (neutrinos might be an example). However, no entity that carries energy or information can exceed this limit. Moreover, no particle that have mass can actually reach speed of light, no matter how much or how long it is accelerated. 

Both of these postulates have been tested and no exceptions have ever been found.