1. A method comprising: identifying, by one or more processors and as a keyword candidate, a first word included in one or more of previously submitted queries; creating, by the one or more processor, a pair that includes the first word and a second word that differs from the first word, the second word being descriptive of a particular category of a plurality of categories; calculating, by the one or more processors, a first value based on a first frequency that the first word appears in the previously submitted queries; calculating, by the one or more processors, a second value based on occurrences of the pair in a collection of documents, one or more documents, in the collection of documents, being classified in the particular category based on occurrences of the second word in the one or more documents, and the second value being calculated based on a second frequency that the first word appears in the one or more documents classified in the particular category; determining, by the one or more processors and based on the first value, that the first value satisfies a first threshold; determining, by the one or more processors and based on the second value, that the second value satisfies a second threshold; determining, by the one or more processors, that the first word is a keyword for the particular category based on the first value satisfying the first threshold and the second value satisfying the second threshold; and associating, by the one or more processors, the first word with the one or more documents based on determining that the first word is the keyword for the particular category.