A Windows service is an application that usually serves a core operating system function running in the background and has no user interface. The Windows operating system makes use of these applications or services to do what an OS does, such as manage network connections, play sound, provide file system functionality, provide security and authentication, display colors and interact with the user through the GUI.
Windows services are the core components of the operating system that manage everything from memory and device management to the management of third-party applications and the user's credentials and preferences, akin to the Unix daemon. These services can be automatically started with the operating system or started manually when specific applications need the service, and can even be disabled. However, core services such the Service Control Manager cannot be disabled and access to it is restricted to the OS alone.
The Windows NT family of operating systems have multiple services, which are grouped into three categories or user accounts: System, Network Service and Local Service. As the names suggest, these services control applications and actions pertaining to their respective categories. There are also third-party services that are installed together with the applications that need them; examples of these are third-party security applications and virus/malware protection software, which install their own constantly running service to actively monitor the system, usually eating up valuable system resources.
The following are some characteristics of Windows services:
Run in a separate Windows session, so are available to all users