Windows 7 introduced support for search federation to remote data stores using OpenSearch technologies that enable users to access and interact with their remote data from within Windows Explorer.
OpenSearch Standard in Windows Federated Search
“The OpenSearch v1.1 standard basically defines a simple file format that can be used to describe how a client should query the web service for the data store and how the service should return results to be rendered by the client.
Windows federated search connects to web services that can receive OpenSearch queries and that can return results in either the RSS or Atom XML format.
A data source that already provides an OpenSearch web service compatible with Windows federated search can be added to Windows Explorer when users open up a Search Connector Description file (.osdx).
To register a new remote data source with Windows federated search, the end-user can open an .osdx file by clicking on a link to one placed on a web site or by opening one provided by someone else on a share or via an email attachment for example.
Doing that creates a .searchconnector-ms file in the Windows “Searches” folder (%userprofile%/Searches) and places a shortcut in the “Links” folder (%userprofile%/Links).”
The OpenSearch 1.1 standard is available here at OpenSearch.org.
Detailed read: Windows 7 Federated Search Provider’s Guide.