Do yourself a favor and read Einstein's paper on special relativity

One of the most famous concepts in science was introduced in a paper with the uninspiring title, "On the Electrodynamics of Moving Bodies." A little reading can give you the basics of relativity, right from the source.

In 1905 Albert Einstein rearranged the universe. That is well-known. What's relatively unknown is the title of the paper in which he did it. This is unsurprising, as the paper, which should be titled, "Dude, This Will Blow Your Mind," is actually given the title of, "On the Electrodynamics of Moving Bodies." I know. Thrilling. Einstein explains the title in the first few lines,

It is known that Maxwell's electrodynamics—as usually understood at the present time—when applied to moving bodies, leads to asymmetries which do not appear to be inherent in the phenomena. Take, for example, the reciprocal electrodynamic action of a magnet and a conductor. The observable phenomenon here depends only on the relative motion of the conductor and the magnet, whereas the customary view draws a sharp distinction between the two cases in which either the one or the other of these bodies is in motion. For if the magnet is in motion and the conductor at rest, there arises in the neighbourhood of the magnet an electric field with a certain definite energy, producing a current at the places where parts of the conductor are situated. But if the magnet is stationary and the conductor in motion, no electric field arises in the neighbourhood of the magnet. In the conductor, however, we find an electromotive force, to which in itself there is no corresponding energy, but which gives rise—assuming equality of relative motion in the two cases discussed—to electric currents of the same path and intensity as those produced by the electric forces in the former case.

In other words, "What's the difference between moving a magnet near a loop of wire, and moving a loop of wire near a magnet?"

This might not tempt too many people, but you can jump ship before you get to the actual electrodynamics. That's in Part II. Part I, the "Kinematical Part," covers most of what you'll find in popular books on special relativity - how the speed of light has to be constant in every frame of motion, how time has to stretch out as a body goes faster, and how lengths have to contract. The language isn't as straightforward as a popular science book, and explanatory diagrams are sorely lacking, but if you want to take a look at a piece of scientific history, with a relatively short word-count, you can't do better than this paper.