Selling the placement of audience segmentation/retargeting cookies on a publisher's site to a data aggregator like Bluekai (typically on a CPM bases) who then sells that aggregated data to DSPs/DMPs for use in display advertising.

If you want to learn about a whole different layer of the ad tech industry, just start perusing some of the questions on Quora for Bluekai (http://www.quora.com/BlueKai) and then do some digging on the companies in the Data Aggregator and Data Supplier buckets of the LUMAscape for display advertising (http://www.lumapartners.com/lumascapes/display-ad-tech-lumas...).