To understand many concepts in relativity one first needs to understand the concept of a reference frame. A reference frame is a system for locating an object's (or event's) position in both space and time. It consists of both a set of coordinate axes and a clock. An object's position and motion will vary in different reference frames. Go back to the example above of the boy and girl tossing the ball back and forth in a train. The boy and girl are in the reference frame of the train; the observer on the bank is in the reference frame of the Earth. The reference frames are moving relative to each other, but there is no absolute reference frame. Either reference frame is as valid as the other.
For his special theory of relativity, published in 1905, Einstein assumed the result of the Michelson-Morley experiment. The speed of light will be the same for any observer in any inertial reference frame, regardless of how fast the observer's reference frame is moving. Einstein also assumed that the laws of physics are the same in all reference frames. In the special theory, Einstein limited himself to the case of nonaccelerating, nonrotating reference frames (moving at a constant velocity), which are called inertial reference frames.
From these assumptions, Einstein was able to find several interesting consequences that are noticeable at speeds close to the speed of light (usually taken as greater than one tenth the speed of light). These consequences may violate our everyday common sense, which is based on the sum total of our experiences. Because we have never traveled close to the speed of light we have never experienced these effects. We can, however, accelerate atomic particles to speeds near the speed of light, and they behave as special relativity predicts.