1. A computer-implemented method for selecting promotional information for display, the method comprising: receiving, by a configured computing system of a content item selection service that communicates with computer systems of a separate online retailer over one or more computer networks, information about search results generated by the online retailer in response to a search request by a user, wherein the search results indicate multiple products available to be acquired from the online retailer and are included in a search results Web page from the online retailer to be displayed to the user on a client device of the user; automatically determining, by the configured computing system of the content item selection service, a product category to associate with the search request by analyzing information about multiple product categories of the indicated products in the search results; sending, by the configured computing system of the content item selection service, one or more electronic communications that have information about one or more additional products to include as part of the search results Web page displayed to the user, wherein the one or more additional products are distinct from the indicated products in the search results and are selected from the determined product category by the configured computing system; determining, by the configured computing system of the content item selection service and based at least in part on the determining of the product category to associate with the search request, to further associate the determined product category with one or more additional Web pages that are displayed to the user on the client device as a result of one or more interactions by the user with the search results included in the displayed search results Web page, including to send one or more additional electronic communications with information about one or more further products selected from the determined product category to include as part of the one or more additional Web pages displayed to the user; and updating, by the configured computing system of the content item selection service, and based at least in part on one or more further interactions by the user with the displayed one or more additional Web pages, an association for the search request from the determined product category to a different product category, for use with later searches by other users using the search request.
1. A computer-implemented method for selecting promotional information for display, the method comprising: receiving, by a configured computing system of a content item selection service that communicates with computer systems of a separate online retailer over one or more computer networks, information about search results generated by the online retailer in response to a search request by a user, wherein the search results indicate multiple products available to be acquired from the online retailer and are included in a search results Web page from the online retailer to be displayed to the user on a client device of the user; automatically determining, by the configured computing system of the content item selection service, a product category to associate with the search request by analyzing information about multiple product categories of the indicated products in the search results; sending, by the configured computing system of the content item selection service, one or more electronic communications that have information about one or more additional products to include as part of the search results Web page displayed to the user, wherein the one or more additional products are distinct from the indicated products in the search results and are selected from the determined product category by the configured computing system; determining, by the configured computing system of the content item selection service and based at least in part on the determining of the product category to associate with the search request, to further associate the determined product category with one or more additional Web pages that are displayed to the user on the client device as a result of one or more interactions by the user with the search results included in the displayed search results Web page, including to send one or more additional electronic communications with information about one or more further products selected from the determined product category to include as part of the one or more additional Web pages displayed to the user; and updating, by the configured computing system of the content item selection service, and based at least in part on one or more further interactions by the user with the displayed one or more additional Web pages, an association for the search request from the determined product category to a different product category, for use with later searches by other users using the search request. 2. The method of claim 1 wherein the search request includes one or more search terms, and wherein the method further comprises storing a mapping of the one or more search terms to the determined product category, and using the stored mapping with the later searches by the other users that use the search request including the one or more search terms.