Simply put, the difference between analog and digital is DIGITAL uses 2 states of circuit activity to determine information - it is either on or off, one or zero. ANALOG uses a spectrum of values between two limits, in other words if digital is zero volts and five volts then analog can derive a value for data from any voltage from zero to five. so the analog system is infinitely variable and can pass much more data per instant than can the digitalsystem. The problem with analog is the ability to accurately determine the exact value of the data which was originally transmitted, since noise and distortion can affect the actual voltage measured at the reciever, so analog systems do not usually operate over more than a few well separated voltage levels within the given range, to try to eliminate errors based on voltage shifts or noise. a simple analog computer can be seen in a servo control system for an electric motor where a given input speed is compared to the actual speed and the analog "computer" changes the drive to the motor to match the demand speed. Analog computers do not have to be electrical either - a servo system can be made with weights on a shaft which move when the shaft spins, this is a simple gyroscope. the moving weights by connecting mechanically to a control brake or throttle cause a change in speed or limit the speed in the mechanical system. Analog computer systems once exsisted in a simple form, but were generally abandoned as digital systems were developed. A fine example of a more complex analog computer used today is an automatic transmission which uses transmission fluid under pressure and engine vaccuum to make decisions about when to change gears and which gear to pick.

http://www.answerbag.com/q_view/987114