Visual studio offers 2 templates for adding WCF.
The following quote from WCF Visual Studio Templates explains the 2 types:
“AJAX enabled WCF Service Item Template
This template exposes an AJAX control as a WCF service. For more information on AJAX controls, see the AJAX control documentation.
Silverlight-enabled WCF Service Item Template
This template creates a Web service that provides data to a Silverlight client or front-end. The template can be added to a Web site or Web application project to create a WCF service, which includes service code and configuration that support communicating with a Silverlight client. You can then use Add Service Reference to add a client proxy of the service to the client, and exchange data between the Silverlight client and the Silverlight-enabled WCF service.
To access this template, right-click a Web site or Web application project in Solution Explorer, click Add a new item, and click Silverlight-enabled WCF Service.
Visual Basic Note:
The Silverlight-enabled WCF Service exposes a basicHttpBinding endpoint without enabling any security settings. Therefore, information about the service can be obtained by all clients that connect to this service. Messages exchanged between the service and the client are also not signed or encrypted. To secure the endpoint properly, you should use ASP.NET authentication, HTTPS or other mechanisms. “
Schleichermann has an excellent blog on this topic: .Net: Difference Between WCF Service and Silverlight-Enabled WCF Service