An ad network, or online advertising network, is a business that links advertisers with websites or apps that want to show ads. Its main job is to find ad space that is available from a lot of publishers and match it with advertisers who want to reach certain groups of people.
Ad networks used to mean networks for TV or print ads, but now they mostly mean networks for online ads. The main difference between traditional and online ad networks is the technology they use. Online networks use a central ad server to send, follow, and measure ads in real time. Compared to older, non-digital media, this lets you target better, track performance better, and get more detailed reports.