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The special theory of relativity is a theory of the structure of spacetime, which asserts that 1) the laws of physics are the same for all observers in uniform motion relative to one another and 2) the speed of light in a vacuum is the same for all observers, regardless of their motion of the motion of the light source. This leads to bizarre consequences. For instance, imagine moving on a train going 100 mph next to another train moving in the same direction at 100 mph. From your perspective, the train next to you looks almost still. But imagine being on a train moving at the speed of light, while next to a light beam. Instead of the light beam looking like it's staying still, it moves ahead of you at the speed of light. 

Several of the consequences that follow from the special theory of relativity are 1) time dilation (clocks moving near the speed of light operate more slowly than stationary clocks), 2) length contraction (objects moving near the speed of light appear to be contracted in the direction of motion), 3) relativity of simultaneity (things that appear to happen at the same time to stationary observer A may appear to happen at different times to moving observer B), 4) mass-energy equivalence (mass and energy are essentially the same thing and transmutable into each other).