Windows Server Service
Specifies a service that is installed onto the server to perform major actions. A Windows server service runs in a separate process space from the SharePoint instance.
Real World Example
You might have a translation service that waits for external calls and, upon receiving a request, does automatic language translation on the documents, saving the translated versions to a document library.
A Windows server service is one of the core ways to perform continuous server-side operations outside of the SharePoint process space. Windows server services are normally installed outside of the SharePoint infrastructure, and usually have a much richer range of capabilities than code that is directly tied to the SharePoint page-processing lifecycle.
Custom Windows server services have a comparatively high cost compared to other SharePoint customizations, with regard to initial and ongoing operations (potential server load) and management (such as password changes). As such, you should install custom Windows server services only in rare cases.
You can install Windows server services onto a server by using the SharePoint solution deployment mechanism as long as you deploy the solution as an application.