An ad network aggregates ad inventory from publishers and sells it to advertisers. It acts as a broker between the two parties. Ad networks make the process convenient by matching advertiser demand with publisher supply. When a match occurs, the ad details are sent to the publisher to display. This allows advertisers access to premium inventory at scale, while publishers can generate revenue immediately by listing their inventory. Major ad networks include Google AdSense, PropellerAds, and Adcash. [/SUMMARY]