14. The computer storage device of claim 13 , further comprising instructions that when executed cause the computer to perform operations further comprising: computing, for each of the terms in the particular cluster and each of the terms in the co-relevant cluster, a relevance score indicative of a measure of relevance of the term to the specified text; and providing the terms having a relevance score that satisfies a relevance threshold.
13. A computer storage device encoded with a computer program comprising instructions that when executed operate to cause a computer to perform operations comprising: accessing selection data for a plurality of advertisements, the selection data specifying search queries for which the advertisements were presented and user selections of the advertisements in response to the presentations; creating clusters of terms and corresponding advertisements based on the selection data, each of the clusters including multiple corresponding advertisements and each of the corresponding advertisements in each cluster having a term vector that is within a threshold distance of each other term vector for other corresponding advertisements in the cluster, each term vector for a corresponding advertisement specifying the search queries for which the corresponding advertisement was both presented to a user and selected by the user, the term vector also specifying advertiser-designate keywords for the corresponding advertisement that triggered presentations of the corresponding advertisement, wherein at least one of the advertiser-designated keywords is not included in the search queries, and wherein creating the clusters comprises determining cluster vectors for the clusters, each cluster vector for a respective cluster being an aggregate representation of term vectors for each of multiple corresponding advertisements in the respective cluster; computing-similarity measures between pairs of the clusters, each similarity measure for a pair of clusters being based on a distance between a cluster vector for a first cluster of the pair and a cluster vector for a second cluster of the pair; receiving a request for data identified as relevant to specified text; in response to the request: identifying, from the clusters, a particular cluster that includes a term matching the specified text; identifying, from the clusters, a co-relevant cluster for the particular cluster, the co-relevant cluster being identified based on the computed similarity measure between the particular cluster and the co-relevant cluster meeting a threshold similarity measure, the co-relevant cluster being a different cluster than the clusters that include the term matching the specified text; and providing data from the particular cluster and data from the co-relevant cluster.