The diagram to the right shows what a basic differential drive looks like. Bevel gears placed in the central gear housing allow the axles to turn independently, enabling the robot to turn in place, and maneuver pretty easily.
The following chart is adapted from "Building Robots with Lego Mindstorms" by the Ferrari Brothers and displays the different behaviors of a robot in accordance to its wheels (powered by the differential drive).
Using different combinations of speed and direction, the robot has the ability to cover any amount of space, and turns at any angle. All this in addition to being very easy to implement.
I assembled an example of a standard drive, showing how you can use the bevel gears inside the housing to attach the axles, and connecting it to an RCX is as easy as sliding two beams through it.
As always, I encourage you to send me your designs, and contact me with any questions/comments/concerns. Happy building!