The Theory of Relativity has indeed made a remarkable impression on the world of modern physics.  Before Einstein, time was thought of as an unchanging quantity, independent of the observer and physical location.  One second on Earth was one second on Mars, one second on Vega, one second in the Horsehead Nebula, one second anywhere.  Einstein, however, came to disagree with this standard viewpoint of time.  According to Einstein, time and space are not two quantities independent from one another, but rather intertwined with one another in a feature called spacetime geometry. 

            Before we discuss the consequences of Special Relativity, we have to understand the concept of how an observer located in an inertial reference frame describes an event (an occurrence that can be described by four coordinates [three spatial and one time based]).  Different observers in different inertial frames have the capabilities of describing the same event (only with different coordinates).  The reference frame that is utilized to describe events tends to be dependent on a coordinate grid and synchronized clocks for time measurement.  In order to synchronize the clocks, light can be used.  For instance, if we were to station an observer at the origin with his or her clock and require him or her to send out a light signal at t = 0, the light message will take a time of r/c to reach the second clock where r is the distance from the origin.  The clocks will be classified as synchronized if the second clock reads a time of r/c at the instant the light pulse reaches it. 

Special Relativity has its basis in two postulates:  (1) All the laws of Physics are consistent in all inertial reference frames and (2) the speed of light in a vacuum  (c = 3.00 X 10⁸ m/s) is the same value in all inertial frames, regardless of the velocities of observers or velocity of the light-emitting objects.  So in other words, say we have a truck moving at a constant velocity of 20 m/s and a man is situated in the back of the truck with a flashlight, who initiates a pattern-like flashing.  In Newtonian mechanics, the speed of the light in this case would be a simple addition of velocities (20 m/s + c), however, according to the speed of light postulate, the speed of light in this case (or any case) will always be c.    The concepts of length, time and simultaneity differ between relativistic mechanics and Newtonian mechanics.  In relativistic mechanics, there is no such existence of an absolute length or an absolute time since length and time depend on the frame of reference of the observer.