In general, a servo
is any device that can be made to turn to a specified angle or number of rotations.
It is most commonly found in radio-controlled vehicles. Usually, it consists of a small motor and a small potentiometer that tells it how far it is turned. When it receives a signal which is usually pulse width modulated and usually comes from a receiver, it turns to a particular position. Usually attached to the top of the servo is a servo horn, which
is then used to turn something else. Usually the receiver is connected to an antenna, which receives the signal from a transmitter. The receiver and the transmitter must use the same radio frequency (usually in the 30MHz or 40MHz range). This is ensured by using the same frequency crystals to select which frequency the receiver and transmitter use.
For example, in a radio-controlled car, there are typically two servos: one for steering, one for throttle. When the steering servo is turned, it causes the wheels of the car to turn. When the throttle is turned, it might connect to the carburetor of the engine, causing more fuel to flow and hence, cause the engine to go faster. By changing the controls on the transmitter, you can thus make the car turn and go fast or slow.
Two important characteristic of servos are speed (how fast they go to a fixed position) and torque (how hard they can push). There is also the issue of size. They range from being, say 5cm x 2cm x 5cm to being huge 20cm beasts. There are a number of other important characteristics, like turn angle ... most common servos will only rotate through 90 degrees, but some custom servos, e.g. those used in sailing can turn close to 360. Well-known servo manufacturers include Futaba, Hitec, Sanwa and JR.