This instructable describes how to integrate hobby servos (the kind used in RC planes, cars, etc.) into your microcontroller projects.
Step 1: How Servos are Different from Regular Motors
In a regular DC motor, the amount of torque the motor exerts on the shaft is proportional to the amount of current flowing through the motor. A simple way to control it is by varying the voltage across the motor; more voltage means more current which means the motor pushes harder against its load which means the shaft turns faster.
When using a servo, however, you don't control torque or velocity. Instead, you specify what angle you want the shaft at. In other words, you have positional control of the motor.
Inside a servo is a traditional DC motor, a potentiometer (variable resistor), and control circuitry. The potentiometer is connected to the motor such that when the motor shaft turns it also turns the potentiometer. The controller can then measure the voltage at the center pin of the potentiometer and get an indication of the shaft's position. The controller receives a signal (see next step) from the user that sets a desired position. The controller compares the desired position to the current position of the motor and uses that information to turn the motor in a direction that minimizes the error.
The way this works in practice is you specify the angle you want the shaft at using your PIC, the shaft turns to that position, and then holds there. The further it gets pushed away from that position, the harder it tries to turn back. Hobby servos are usually geared way down, so even a wimpy $15 or $20 one can hold its position reasonably well.