Perhaps the most well-known "result" from the theory of special relativity is the formula E = mc², which defines an equivalence between the mass of an object and its energy.  However, special relativity aims to investigate a much more general problem, of which this famous formula is one outcome.

Special relativity is based on a generalisation of the idea that all motion is relative - that is, one can only judge the state of motion of an object by comparing it with some other reference point or object.  For example, cars move relative to the surface of the earth.  The point is that since motion is relative, it should not matter from what "frame of reference" we observe the universe (i.e. from the earth or from the car), we should always observe the same laws of nature.

Einstein based his theory of special relativity on two basic postulates and then proceeded to determine the consequences of those postulates.  As with quantum physics, the consequences can be surprising and somewhat non-intuitive.

In what follows, the views given here are my own interpretation of special relativity.  I have made no attempt at a rigorous treatment, such as you might find in a textbook about the subject, and I have kept the maths to a minimum.  The following sections are reasonably stand-alone, but you might get the best benefit if you start at the beginning and work your way through

Frames of Reference and Postulates of Relativity

Time Dilation

Length Contraction

Mass-Energy Relationship

The Twin Paradox