1. A computer-implemented method for use by an ad server system including at least one computer on a network, in an environment including a client device and the ad server system, the computer-implemented method comprising: a) accepting, by the ad server system, document relevance information including at least one concept extracted, by the client device, from content of a document requested by the client device, wherein the document relevance information is generated and sent to the ad server system by at least one of (A) a browser application on the client device, (B) a browser plug-in on the client device, and (C) a browser toolbar on the client device, wherein the document relevance information is automatically extracted by the client device from the document requested; b) determining, by the ad server system, at least one ad relevant to content of the document using at least the accepted information to find matching serving constraints stored in association with ads; and c) sending, by the ad server system, the at least one ad determined to the client device.