1. A delivery system for selecting advertisements for presentation to a user in response to a user search query, the system comprising: a keyword server for parsing the user search query; an index server for receiving the parsed search query, the index server comprising an index of advertising phrases and pre-filtering components for comparing index entries to the parsed user search query in order to discard non-matching index entries and locate matching index entries; and a listing server for sorting through the matching index entries located by the index server and further filtering the matching entries for retrieval and presentation of associated advertisements to the user.