As a website publisher, you would get enrolled in lots of Ads network as publisher to get ads on your website. This helps in generated some revenue out of the content of your website.
Google Adsense is one of the largest such Ads network used by millions of website owners.
However, at times, Google Adsense may display Ads from its third-party partner network as well. In my case, I stumbled upon couple of Ads slots stating that the ads were by Criteo. I was confused about that as I had never enrolled into Criteo Ads network as publisher but still my website was displaying ads from it.
Then I googled and realised that Google Adsense has the feature to display non-Adsense ads which was happening in my case.
If you wish to block that from happening, go to : AdSense dashboard > Blocking controls > All sites > Manage Ad networks.