What is Advertising Network (Ad Network)? – Definition from Techopedia

An advertising network (ad network) is an online business that specializes in matching up advertisers to websites looking to host advertisements. Advertising networks work as brokers for both suppliers (sites with content that can host ads) and buyers (the advertisers). Joining an ad network frees sites from having to set up their own ad servers and invest in tracking software.

Techopedia Explains Advertising Network (Ad Network)

An advertising network essentially brokers advertisement campaigns by finding sites with content inventory that matches the needs of advertisers looking to run ads. The downside of ad networks is that a site must give up some control of its advertising. This means it may be more difficult for the site to be selective of the ads shown to users, or ensure they are getting the best possible ad revenue arrangement.