27. The volatile or non-volatile computer-readable storage medium of claim 18 , wherein the category is associated with a set of related nodes, wherein the one or more sequences of instructions, when executed by the one or more processors, further cause the one or more processors to perform: determining a set of related terms, each term of the set of related terms associated with a node of the set of related nodes; determining that a query is associated with at least one term of the set of related terms; in response to determining that the query is related to the at least one term of the set of related terms, storing a second information that indicates the query is associated with at least one other term of the set of related terms.
18. A volatile or non-volatile computer-readable storage medium storing one or more sequences of instructions which, when executed by one or more computing devices, cause the one or more computing devices to perform: associating a category to a set of nodes of a graph by: determining a first node that represents a first term that is in the category; locating a second node associated with the first node based at least in part on a first degree of cross-reference between the first node and the second node, the second node representing a second term, wherein the first degree of cross-reference is based at least in part on a frequency by which the first term appears in a set of documents with the second term; locating a third node associated with the second node based at least in part on a second degree of cross-reference between the second node and the third node, the third node representing a third term, wherein the second degree of cross-reference is based at least in part on a frequency by which the second term appears in a set of documents with the third term; based at least in part on both (a) the first degree of cross-reference between the first node and the second node, and (b) the second degree of cross-reference between the second node and the third node, determining whether or not the third term is in the category; in response to determining that the third term is in the category, storing information that indicates the third term is in the category.