1. A method for characterizing a document with respect to clusters of conceptually related words, comprising: receiving the document, wherein the document contains a set of words; selecting candidate clusters of conceptually related words that are related to the set of words; wherein the candidate clusters are selected using a model that explains how sets of words are generated from clusters of conceptually related words; and constructing a set of components to characterize the document, wherein the set of components includes components for candidate clusters, wherein each component indicates a degree to which a corresponding candidate cluster is related to the set of words.