Business Data Connectivity service(A shared service in Windows SharePoint Services and SharePoint Server that provides a means for storing, securing, and administering external content types and related objects)
WCF service(An application service such as authentication, profile properties, or roles, that can be accessed by any non-ASP.NET application that can read and send messages in SOAP format. WCF services enable different types of applications to share common data, such as user credentials for logging in)
