25. A non-transitory computer-readable medium encoded with a computer program comprising commands that, when executed, operate to cause one or more computers to perform operations comprising: providing, to a client device, information for presenting a search engine user interface for a third party content provider; receiving, from the client device, a search query entered using the search engine user interface; processing the search query to determine a context identifier for a context file; retrieving a context file from the third party content provider identified by the context identifier; modifying the search query using one or more commands to produce a context processed search query, wherein the one or more commands are contained in the context file; providing the context processed search query to a search engine; obtaining, from the search engine, context processed search results responsive to the context processed search query; and providing the context processed search results to the client device.
25. A non-transitory computer-readable medium encoded with a computer program comprising commands that, when executed, operate to cause one or more computers to perform operations comprising: providing, to a client device, information for presenting a search engine user interface for a third party content provider; receiving, from the client device, a search query entered using the search engine user interface; processing the search query to determine a context identifier for a context file; retrieving a context file from the third party content provider identified by the context identifier; modifying the search query using one or more commands to produce a context processed search query, wherein the one or more commands are contained in the context file; providing the context processed search query to a search engine; obtaining, from the search engine, context processed search results responsive to the context processed search query; and providing the context processed search results to the client device. 30. The non-transitory computer-readable medium of claim 25 , wherein the commands comprise a command that specifies a document collection in which to conduct a search.