You're a hip, savvy Internetter; we know you don't have a lot of time. Here's the skinny: All three alliances want to control Cyrodil, the central part of the continent that houses the very heart of Tamriel. The Tharns that rule Cyrodil are weak, and each alliance thinks that it has the power to steer Tamriel in the proper direction.
Skip below the cut to watch the video to learn about all the intricate details.
[Thanks to groovy webizen Paul for the tip!]