Display inventory on the Display Network is published by AdSense or:
The DoubleClick Ad Exchange
Display inventory on the Google Display Network can come from one of two sources: AdSense or the DoubleClick Ad Exchange. DoubleClick Ad Exchange is a real-time marketplace partnered with the Google Display Network for buying and selling advertising. It represents another market for advertisers to bid on display advertising across the Internet. When you have a campaign that targets the Display Network, you automatically have access to Ad Exchange publisher sites that comply with AdWords’ guidelines.
As long as you’ve targeted the Display Network for your campaign, your ads can appear on Ad Exchange publisher sites, in addition to those available through Google AdSense .
Read more here: https://support.google.com/adwords/answer/2472739?hl=en