1. A method for targeting advertisements to users, the method comprising: using at least one computer system to perform method steps comprising: for a plurality of advertisers: receiving explicit criteria from a plurality of advertisers; providing search results to each of the plurality of advertisers responsive to the explicit criteria; receiving feedback from each of the plurality of advertisers about the search results; determining implicit criteria about which users the plurality of advertisers wish to target based on the feedback received from the plurality of advertisers, the implicit criteria indicating hidden preferences of the advertisers identified from the feedback from each of the plurality of advertisers; for a plurality of users: receiving explicit user criteria providing information about each of the users, the criteria providing information about which advertisements the users should view; providing the users with search results using the explicit user criteria for one or more searches conducted by each of the users; receiving user feedback regarding the search results; determining implicit user criteria based on the feedback provided by the users; creating a plurality of advertiser/ad profiles each using the explicit and implicit advertiser/ad criteria for a given advertiser/ad; creating a plurality of user profiles each using the explicit and implicit user criteria for a given user; comparing the advertiser/ad profiles and the user profiles to generate relevance scores and determine matches between a particular advertiser/ad profile and a particular user profile; determining which of the advertisements to present to each of the plurality of users based on placement factors including computing relevance of each of the advertisements to each of the plurality of users, computing value of each of the plurality of users to each of the advertisers based on the relevance of each of the advertisements to each of the plurality of users, and computing a price for presenting the advertisement to each of the plurality of users; and presenting to each of the plurality of users the advertisements for which a positive determination to place the advertisement is made.
1. A method for targeting advertisements to users, the method comprising: using at least one computer system to perform method steps comprising: for a plurality of advertisers: receiving explicit criteria from a plurality of advertisers; providing search results to each of the plurality of advertisers responsive to the explicit criteria; receiving feedback from each of the plurality of advertisers about the search results; determining implicit criteria about which users the plurality of advertisers wish to target based on the feedback received from the plurality of advertisers, the implicit criteria indicating hidden preferences of the advertisers identified from the feedback from each of the plurality of advertisers; for a plurality of users: receiving explicit user criteria providing information about each of the users, the criteria providing information about which advertisements the users should view; providing the users with search results using the explicit user criteria for one or more searches conducted by each of the users; receiving user feedback regarding the search results; determining implicit user criteria based on the feedback provided by the users; creating a plurality of advertiser/ad profiles each using the explicit and implicit advertiser/ad criteria for a given advertiser/ad; creating a plurality of user profiles each using the explicit and implicit user criteria for a given user; comparing the advertiser/ad profiles and the user profiles to generate relevance scores and determine matches between a particular advertiser/ad profile and a particular user profile; determining which of the advertisements to present to each of the plurality of users based on placement factors including computing relevance of each of the advertisements to each of the plurality of users, computing value of each of the plurality of users to each of the advertisers based on the relevance of each of the advertisements to each of the plurality of users, and computing a price for presenting the advertisement to each of the plurality of users; and presenting to each of the plurality of users the advertisements for which a positive determination to place the advertisement is made. 4. The method of claim 1 , wherein the plurality of advertisers are job candidates placing an advertisement before a job provider and the plurality of users are job providers conducting a search for job candidates to fill job openings.