The H-Bridge
One of the first things you will have to figure out when building robots is how to make it go forward, reverse, turn left, turn right and stop. The H-Bridge Motor Controller is the answer. Here’s a diagram of what an H-Bridge looks like:
You can see that you have 4 switches. If you turn the High Side(left) and the Low Side(right) on, the motor will rotate in one direction. If you turn the High Side(right) and the Low Side(left) on, the motor will rotate in the opposition direction. This happens because you are causing the current to flow through the motor in one direction or the other depending on which switches you have turned on.
It would be impractical if you had to sit there turning switches on and off to make your robot move, so a good solution would be to make an elecgtronic circuit that will accomplish the task automatically by following commands from a micro-controller.
A schematic diagram of an H-Bridge Motor Controller adds some resistors, transistors, and diodes as well as an interface to your micro-controller that will turn the high side and low side combinations on for you. 
This circuit would require you to use for input/output lines on your micro-controller in order to make the motor move in the direction you want based on the programming of the micro-controller.
The resistors are there to prevent the full current from the batteries from smoking your transistors. You only need enough current to turn the transistors on. The diodes are there to prevent surges of current from flowing backwards into the motor when it changes directions.
The inputs from your microcontroller, either a logical 1 or a logical 0 willl controll the direction of the motor and cause it to either go forward or reverse.
