1. A computer implemented method of administering a bidding language for online advertising, the method comprising: providing, using a computer, an expressive tree structure that allows an advertiser to specify an ad campaign in a plurality of nodes, the nodes comprising: at least one value attribute corresponding to an advertiser-specified value associated with a grouping of consumers represented by a node, at least one volume constraint corresponding to limit on exposure of the ad campaign to a grouping of consumers represented by a node, wherein the volume constraint is determined by the advertiser based on a desired focus of the ad campaign; providing, using a computer, a bidder feedback mechanism for the advertiser, by receiving, from the advertiser, at least one target impressions value from the advertiser corresponding to at least one grouping of consumers represented by at least one node, transmitting, to the advertiser, at least one recommended efficient allocation comprising a recommended value attribute and a recommended volume constraint corresponding to at least one node, wherein the recommended efficient allocation is computed based on the target impressions value, the value attribute, the volume constraint, and an inventory forecast; and adjusting, in a computer, the ad campaign based on the recommended efficient allocation, receiving, at a computer, the ad campaign that is written in the expressive bidding language; translating, using a computer, the ad campaign into scripts; and using, in a computer, the scripts to access appropriate inventory in an ad inventory database.