With Windows 7 Federated Search users can access and interact with both internal and external contents and data from within Windows Explorer. Much has been written, for example, David Chou’s blog, about what the Federated Search is and how to search web contents using custom search connectors to web sites. And you can download some search connectors from sites such as redmondpie and thewindowsclub.
However, developers can enable Windows Federated search on backend data stores. The architecture diagram from MSDN illustrates how Windows Explorer communicates with the backend data store via OpenSearch web service.
You can find more detail on this MSDN white paper on “Connecting Your Web Service in Windows Federated Search.”