47. The storage device of claim 43 wherein the operations further comprise: identifying one or more additional electronic documents wherein each of the additional documents was referred to by one or more respective user selected search results wherein each user selected search result was responsive to a respective query; categorizing one or more of the identified additional documents by associating the additional document with a respective category that is a hierarchical classification of the additional document; and updating the category graph based on the additional document categorizations.
47. The storage device of claim 43 wherein the operations further comprise: identifying one or more additional electronic documents wherein each of the additional documents was referred to by one or more respective user selected search results wherein each user selected search result was responsive to a respective query; categorizing one or more of the identified additional documents by associating the additional document with a respective category that is a hierarchical classification of the additional document; and updating the category graph based on the additional document categorizations. 48. The storage device of claim 47 wherein the operations further comprise: associating one or more respective n-grams with each of one or more of the categorized additional documents, wherein the n-grams occur in one or more of the queries of the selected search results that refer to the additional document; and updating the category graph based on the respective n-grams associated with the additional documents.