When you build robots, you will often need precision control.
One way to do this is through feedback.
If you move a limb, and you want it position and a precise location, you would probably employ a method involving a MicroController, a motor and some sort of device to say how far from the target the motor is.
A Servo Motor is precisely these things wrapped up in one package. All you need to do, is tell it where you want it to be. This is normally done by sending some sort of time-based signal using PWM.
There is a common method in robotics of modifying servos to get constant speed from them for drive motors. The reason for doing that is that most microcontrollers have PWM controllers built in - and it is a much better method of controlling motors than simply lowering the voltage or putting a resistor with it.
Decent servos are often found in RC Cars and Futaba seems to be the favourite brand. Hobby magazines for Aircraft and boat modelling often have catalogue sections with many good prices for Servos.
Most servos come in a package with the geared motor and a control circuit with a sensory device. They normally have three connections - two for the power (positive and ground) and one signal line. There are two main varieties - analogue - which use the signals duty ratio to determine their position, and fully digital ones which actually employ a serial protocol allowing for easy digital control, and may even allow a MicroController to read back the position of them.