I made this for a presentation, so I am including a short general introduction here:
Special theory of relativity claims that objects that travel with a constant velocity with respect to some observer behave very strangely. If the speed is comparable with the speed of light, the observer can see that the object has shrinked in the direction of motion. Also if the observer was able to weigh it, he would find out that the object's mass increased. And if the object was moving, the observer would see that the object's time has slowed down. Another observer sitting on the object sees no difference, because the object is not moving with respect to him. It is all about relative speed, hence the name.
In this little Flash application you can toy with the speed of the famed U.S.S. Enterprise and see about the consequences.