The servo is a small motor that turns an arm (or wheel) 180˚ Backwards and forwards. They are used a lot in remote control toys. They can be inexpensive depending on what you want to use it for.
An Arduino Uno can drive up to 8 servos according to their documentation. It is driven by attaching it to a digital pin. The other two are connected to ground and the 5v supply.
It is usually a good idea to either use a separate power supply or a capacitor when using it (or certainly more than one). The sudden power surge can damage the Arduino if not careful. This is not likely to happen but could happen.