Every time a user logs on the NT machine, a desktop is immediately assigned to him. Any process or thread started by the user is attached to this desktop. Multiple desktops can exist, but only one desktop can be active at a time. These desktops each have threads assigned to them. Windows can be created in each of these threads, but only those windows in threads assigned to the active desktop are visible.
Windows services are processes that exist without any desktops and run in the background (similar to daemon processes in UNIX). They can be started before any user logs into the system. After the operating system is initially loaded, it immediately begins loading the services that are configured to automatically start. The user can log on to the system before all automatic services are loaded, but the user is not presented with a desktop until all the automatic services have finished loading. After logging on, the user can start and stop the services. When the user logs off, the desktop is closed and all the processes assigned to his desktop are also closed. But any services that exist when the user logs off are not closed and remain alive and running. They are often loaded at boot up, before any user logs in, and are often independent of any specific user logged on at the time. In fact, a service has its own login session. A service can also be launched manually by a user. Although it’s possible, Windows Service typically has no user interface.