A standard hobby RC servo motor is designed to rotate through only 180 degrees. They are most often used in RC applications (like steering). The servo motor controlling is done with servo control pulses (1-2 ms pulse at around 50 Hz rate). More details on the control signals can be found at Servo protocol web page.
If you wish to use a servo motor for a robot drive system, you want a motor that turns more than 180 degrees. Continuous Rotation Servo is an useful tool for robotics and basic movement projects.
It is possible to modify a normal RC servo motor (at least most of them) so that it will rotate through 360 degrees and beyond and in either direction. It gives you a nice geared motor which is controllable by PWM signals. Modifying Hobby Servo Motors for Continuous Rotation paper details how to make this modification on a Futaba S3003 servo motor, but the principles should work on most other hobby servos. In this modification you need to do a little bit soldering (replace potentiometer with two resistors) and some mechanical modification (remove the end stopper from servo).
Modification of a Futaba S3003/S3004 Servo for Continuous Rotation shows another way to do the modification, here you just make two mechanical modifications: cut the end stopper and potentiometer shaft. There are several YouTube video that show you how you do the modification: Futaba servo motor modification, How to Convert a Servo to Continuous rotation and How to: Mod servo for continous rotation.
I have made some playing with the idea and found that those modification principles work. I opened one Futaba S3003 servo and played with it without doing all the modifications.
The modified servo motor can be an useful tool for robotics application, because it is small and can be controlled with anything that can control normal RC servos (for example normal RC transmitter+receiver pair or for example Arduino board). This kind of modified servo motor works so that when the servo is set to neutral position (center), the motor will be stopped. When you move the position off from it, the motor starts rotating to direction controlled with the change direction. With change from central position first the motor typically start rotating slowly, and with somewhat more off the center the motor reaches the full speed.
The modified servo is not ideal controlled motor, because the exact position which is “neutral” (motor is stopped) is quite small and typically varies somewhat from servo to servo (needs tweaking to get things so what motor stops when you want). Also the speed control is not very accurate.
Traditional way to control rotating motors with RC signals would be to use an electronic speed control (ESC) that takes in RC servo pulse controls signals and sends controlled amount of power to the controlled electrical motor (being it traditional DC motor or brush-less DC motor). Regardless of the type used, an ESC interprets control information not as mechanical motion as would be the case of a servo, but rather in a way that varies the switching rate of a network of field effect transistors that send the power to the motor. ESCs designed for sport use in cars generally can run motors on both directions (have reversing capability).