A service is an offer to perform a particular function. In the case of file serving, a service may be the transmission of a file. In the case of web serving, a service is the processing of an URL request and the transmission of the appropriate HTML document. In the case of multimedia serving, a service is the transmission of a video/audio stream.
It should be noted that each instance of “service” consists of two parts. The first part is a request (for service), and the second part is a response (to the request).