1. A computer-implemented method for providing advertisements to a client computer, the method comprising: defining a plurality of referral labels, wherein each referral label is assigned to one of a plurality of publishers, wherein different publishers of the plurality of publishers are assigned different referral labels, and wherein at least some of the referral labels represent referral relationships between different publishers; receiving a request originating from a client computer for a first advertisement related to a first content item published by a first publisher of the plurality of publishers, the request including a first referral label; identifying a first advertisement that is associated with a second referral label, the first advertisement being provided by a second publisher of the plurality of publishers, wherein the first referral label and the second referral label represent a referral relationship between the first publisher and the second publisher; and transmitting the first advertisement for presentation to a user by the client computer.