8. A computer system comprising a processor for executing program instructions coupled to a memory for storing the program instructions, wherein the program instructions are program instructions for retrieving documents from a collection of documents, and wherein the program instructions comprise: program instructions for receiving a search request including search terms descriptive of documents in the collection; first program instructions for matching the search terms to descriptive terms of entries in multiple dictionaries to determine multiple subjects specified by the search terms, the multiple dictionaries have an associated unique subject, wherein entries in the multiple dictionaries contain descriptive terms, wherein at least some of the descriptive terms are present in two or more of the multiple dictionaries, and wherein the first matching generates scores indicating the relative strength of a relationship between the search terms and the multiple subjects; second program instructions for matching multiple subjects determined by the first matching to documents in the collection of documents to obtain a subset of the collection of documents that are relevant to multiple subjects; program instructions for maintaining records of hierarchy of classification for the entries within the multiple dictionaries for the multiple dictionaries, wherein the hierarchy records encode or store affinity values showing a strength of relationship between the entries within a corresponding dictionary; program instructions for adjusting a result of the first matching or the second matching using the records of hierarchy of classification for the multiple subjects determined by the first matching; and program instructions for returning at least a portion of the subset of the collection of documents obtained by the second matching.
14. The computer system of claim 8 , wherein the program instructions for maintaining a record of a hierarchy of classification of the entries maintain a tree structure having hierarchical levels corresponding to levels of classification, wherein a highest level of the hierarchy of classification is the subject associated with the dictionary and wherein the lowest level of the hierarchy of classification contains descriptive terms that do not describe a further sub-classification, with terms corresponding to sub-classifications of the subject disposed between the highest level of the hierarchy of classification and the lowest level of the hierarchy of classification.