1. A computer-implemented method, comprising: accessing, by a data processing device, 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, by the data processing device, 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 respective 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-designated 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 text of 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, by the data processing device, 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, by the data processing device, a request for data identified as relevant to specified text; in response to the request: identifying, from the clusters, a particular duster 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, by the data processing device, data from the particular cluster and data from the co-relevant cluster.