1. A method for providing information from a corpus of content items to a client, the method comprising: accessing a plurality of content items, in the corpus, into which a plurality of publishers have explicitly inserted a plurality of grouping labels, wherein at least one grouping label of the plurality of grouping labels is included in each content item, of the plurality of content items, by the content publisher, of the plurality of content publishers, that published said each content item; automatically building an index configured to identify, for each grouping label of the plurality of grouping labels, each of the content items in the corpus that includes said each grouping label of the plurality of grouping labels; receiving from a client a request for identification of content items related to a first content item, the request including a first one of the plurality of grouping labels that was extracted from the first content item; accessing the index to identify a second content item having the first one of the plurality of grouping labels included therein; and returning a reference to the second content item to the client; wherein the method is performed by one or more computing devices.