1. A computer-implemented method for providing advertisements to a client computer, the method comprising: defining a plurality of identifiers, wherein each identifier is assigned to one of a plurality of publishers, wherein different publishers of the plurality of publishers are assigned different identifiers; receiving a request originating from a client computer for advertising content; wherein the request includes a first referral label that includes a first identifier that identifies a first publisher of a plurality of publishers, wherein the first publisher published a first content item with which the advertising content is to be presented; based on the first identifier included in the first referral label, searching for an advertisement, of a plurality of available advertisements, that (a) is associated with another referral label that specifies a referral relationship that involves the first publisher, and (b) is published by a publisher other than the first publisher, wherein at least some of the referral labels specify referral relationships between different publishers; based on said searching, selecting a first advertisement that is associated with a second referral label, the first advertisement being published by a second publisher of the plurality of publishers, wherein the second referral label specifies a referral relationship that involves the first publisher; and in response to the request, transmitting the first advertisement to the client computer for presentation, to a user, with the first content item; wherein at least the steps of receiving, searching, selecting, and transmitting are performed by one or more computing devices.