Servos can be electronic or mechanical or a combination of the two. Servos are used in devices that rely on precise operation under varying loads or conditions. In audio, the servo looks at the output signal (via a feedback signal taken from the output of the device) and corrects if the audio output signal varies from it's intended output. For amplifiers, it means that the internal drive circuits will drive harder to make up for losses of any electrical/electronic components in the output circuit. In the examples here, the correction is slow. In electronic circuits, the correction is instantaneous. The example below will help you to understand one type of servo controlled system.

A servo controlled system has a point of reference (generally set by the user), a feedback sensor and a means to correct for errors. In the following demo, the valve assembly is the sensor. The point of reference is the height at which the green arrow (on the valve assembly) is set. If it were set higher, the lever in the demo would be set to stay at an angle instead of parallel to the floor. The device that will correct for an unlevel condition is the hydraulic system.

In the previous demo, the weight was placed on the left side of the fulcrum. If it were set on the right side, the system would still correct as the green valve was pushed from its point of rest. Instead of the hydraulic fluid going into the bottom of the cylinder and pushing up as it did before, the fluid would flow into the top of the cylinder and the cylinder would pull down. As soon as the lever was again parallel to the floor, the the green valve would return to its point of rest and the hydraulic cylinder would stop.

If you've ever used a tractor with a 3-point hitch, you've probably seen a similar type of servo. The control for the height of the 3-point hitch is a servo mechanism. You set the handle to a reference point and the hitch will quickly move to the desired level. To compare the tractor to the device shown here, the servo control on the tractor would be analogous to the entire valve assembly here.

Another very imprecise version of a servo type feedback loop is you when you're driving. If you're trying to maintain a steady speed when driving on hills, you have to keep an eye on the speedometer and vary the position of the gas pedal to maintain the target speed. If you're lucky, you have a 'cruise control' and can set it to maintain the correct speed. The cruise control uses sensors on the transmission or the wheels as the feedback signal to maintain the correct speed. When you set the cruise control, it takes note of the number of pulses per second from its wheel/transmission speed sensor. If the pulse rate drops below a certain point, it will open the throttle a little more. If the rate gets too high, the throttle will be cut back.

Servo Controlled Speakers:
Some subwoofers use servo feedback to virtually eliminate distortion. Velodyne speakers use an accelerometer mounted on the woofer's voice coil former to monitor the speaker's output. If there is any deviation from the desired output of the speaker, the servo circuit corrects. Kenwood has some amplifiers out that allow the amp to monitor the signal at the speaker. This allows the amplifier to correct for losses in the speaker wire.

Other uses of servo/feedback loops are covered on the op-amp page.