a long-running application without a user interface or any visual output
an 'always on' application that is constantly running in the background of your system
an application that does not have a user interface
an application that requires no user interface and can be automatically started at system startup
an application that sits in the background, waiting for you to call it or for some system event to happen
an application which does not have a user interface and runs without human intervention
a process that runs unattended and doesn't have a user interface
a specially designed application that can be started automatically at system boot
A Windows service is an application that starts when Windows is booted and runs in the background as long as Windows is running. It is very similar in concept to a Unix daemon.