Services on stand-alone ArcGIS Servers
ArcGIS Server hosts GIS services. A service represents a geospatial resource—such as a map, locator, or geodatabase connection—made available to client apps.
Services allow you to share the use of resources across clients. You can be sure that each client has the same view of the resource, and you save resources because the server is storing the resources and the clients don't need to have GIS software installed. Instead, the server stores the resource, hosts the service, and does the geospatial work, sending back a result in a common format—such as images or text—to the client.
To learn more about the different GIS Server services types, see Types of services.