9. A system, comprising: a data store storing advertisement selection data for a set of advertisements, the selection data specifying selections of the advertisements from search results pages for search queries; an advertisement management system comprising one or more processors configured to receive specified text and provide advertisements that are relevant to the specified text based on targeting keywords for the advertisements matching the specified text; and an ad-selection analysis subsystem coupled to the data store and the advertisement management system, the ad-selection analysis subsystem including one or more processors configured to perform operations including: creating clusters of terms and corresponding advertisements based on the advertisement 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-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 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 identified as relevant to the specified text, the data specifying at least one additional advertisement that is relevant to the specified text, the additional advertisement being one of the corresponding advertisements from the co-relevant cluster.
9. A system, comprising: a data store storing advertisement selection data for a set of advertisements, the selection data specifying selections of the advertisements from search results pages for search queries; an advertisement management system comprising one or more processors configured to receive specified text and provide advertisements that are relevant to the specified text based on targeting keywords for the advertisements matching the specified text; and an ad-selection analysis subsystem coupled to the data store and the advertisement management system, the ad-selection analysis subsystem including one or more processors configured to perform operations including: creating clusters of terms and corresponding advertisements based on the advertisement 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-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 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 identified as relevant to the specified text, the data specifying at least one additional advertisement that is relevant to the specified text, the additional advertisement being one of the corresponding advertisements from the co-relevant cluster. 10. The system of claim 9 , wherein the ad-selection analysis subsystem is further configured to perform operations including: generating vectors of selection data for each of the advertisements, each of the vectors including weights corresponding to a measure of selections of one of the advertisement when presented with search results pages for the search queries; providing the vectors as input to a clustering algorithm; and receiving data specifying clusters of terms and corresponding advertisements based on the vectors.