an indepdant "background" application that runs wihout a user at the machine (although it does run under some identity)

a process that is started automatically when NT starts up, and runs in the background as long as NT is running

a special type of application that runs on Windows NT

a specific system function that runs in a special area under Windows NT

Work executed by Microsoft's operating system NT; a service can be a software application running or control over a hardware component.