13. A system comprising: a network interface configured to receive structured data describing a content item, the structured data including one or more data elements comprising one or more descriptions and values for the one or more data elements, the structured data indicating a category for the content item; a data storage device storing stored data for the category, the stored data identifying one or more domains; a module configured to form a first query based on data elements in the structured data; a module configured to search at least one of the one or more of the domains using the first query to identify resources associated with the one or more domains; a module configured to determine one or more queries based on data reflecting past search queries, where each of the one or more queries resulted in one or more of the resources being returned as part of a search result; a module configured to determine keywords based on the one or more queries; a module configured to transmit or to store the keywords for use in impression allocation decisions; a module configured to determine an allocation score for the content item based at least in part on comparison of the keywords to a term associated with a request for an impression, where the allocation score is used to determine whether the content item will be selected for the impression; a module configured to select the content item for display in response to the request, based on the allocation score; and a network interface configured to transmit data specifying the content item in response to the request.
13. A system comprising: a network interface configured to receive structured data describing a content item, the structured data including one or more data elements comprising one or more descriptions and values for the one or more data elements, the structured data indicating a category for the content item; a data storage device storing stored data for the category, the stored data identifying one or more domains; a module configured to form a first query based on data elements in the structured data; a module configured to search at least one of the one or more of the domains using the first query to identify resources associated with the one or more domains; a module configured to determine one or more queries based on data reflecting past search queries, where each of the one or more queries resulted in one or more of the resources being returned as part of a search result; a module configured to determine keywords based on the one or more queries; a module configured to transmit or to store the keywords for use in impression allocation decisions; a module configured to determine an allocation score for the content item based at least in part on comparison of the keywords to a term associated with a request for an impression, where the allocation score is used to determine whether the content item will be selected for the impression; a module configured to select the content item for display in response to the request, based on the allocation score; and a network interface configured to transmit data specifying the content item in response to the request. 16. The system of claim 13 , in which at most one resource from each domain in the one or more domains is used to determine the one or more queries.