Service applications work by receiving requests from client applications. They then process these by returning data to the users. Usually, these run in the background with little or no human interference. Web internet services, the File Transfer Protocol (FTP), and email servers are examples of service applications.
A service or application service is required to perform tasks behind the scenes. Certain types of tasks do not necessarily need interaction with the screen or the graphical user interface (GUI). An alarm clock is one example of this type of service; while a previous configuration is required to inform the computer of the alarm time, it is not necessary to keep the application open—you can just set it up and the alarm will be triggered at the right time.
Another example of an application service is the internet...