16. The apparatus as set forth in claim 14 , wherein the category descriptors comprise textual category descriptors and the category profiles are language models comprising multinomial distributions over the words of a textual vocabulary.
14. An apparatus comprising: a digital processing device configured to: generate a hierarchical classifier for a taxonomy of hierarchically organized categories wherein each category is represented by one or more category descriptors, the digital processing device generating the hierarchical classifier by a method including: constructing queries for categories of the taxonomy of hierarchically organized categories, the query constructed for a category c of the taxonomy including a query component constructed from one or more textual descriptors of the category c and at least one of a query component constructed from one or more textual descriptors of an ancestor category of the category c and a query component constructed from textual descriptors of one or more descendant categories of the category c; querying a documents database using the constructed queries to retrieve pseudo-relevant documents; and extracting category profiles for the categories of the taxonomy from at least the pseudo-relevant documents by inferring a hierarchical topic model representing the taxonomy; and classify an input document different from the pseudo-relevant documents using the generated hierarchical classifier.