1. A storage medium storing instructions executable by a processing device to perform a method comprising: generating a hierarchical classifier for a taxonomy of hierarchically organized categories wherein each category is represented by one or more textual category descriptors, the hierarchical classifier being generated by a method including (i) constructing queries from the textual category descriptors representing the categories and querying a documents database using the constructed queries to retrieve pseudo-relevant documents and (ii) extracting language models comprising multinomial distributions over the words of a textual vocabulary for the categories of the taxonomy by inferring a hierarchical topic model representing the taxonomy from at least the pseudo-relevant documents; and classifying an input document using the generated hierarchical classifier.