A physical device that performs an operation on a signal is called as a system. Example: For reducing the noise corrupting a desired information bearing signal, a filter is used. So a filter can be considered as a system. Any operation which changes the characteristics of a signal is called as Signal Processing. The characteristics can be the amplitude, shape, frequency content and phase of a signal.
In science and engineering, most of the signals dealing with these branches are of analog in nature. The analog signals are functions of a continuous variable as space or time. Devices such as amplifiers, filters, frequency analyzers etc are required to process such type of signals. These devices respond to the continuous variation of the instantaneous amplitude of the input signal. These devices can change the characteristics of a signal. These devices are also useful in extracting some desired information from the signal. Thus in these cases, it is clear to say that the analog signal has been processed. An analog signal processing system can be defined as the system that processes an analog signal.