Ad network is a place to store ads, and they are also accountable for collecting inventory from publisher websites and then selling those to diverse advertisers.
The purpose of the networks is to connect advertisers and app publishers. Advertisers will offer their ads to the ad network with the goal to promote their brand and product. After that, app publishers will receive the payment by presenting these advertisements to their app users. With the purpose of gaining permission to access the network, app publishers must integrate an SDK if they wish to proceed to the next step.
There will be confusing points in distinguishing ad exchange and ad network, as they are both platforms which contain ads. So if you want to find the difference, please read our comparison here.