Banner ad networks are platforms that connect advertisers with websites that want to display ads. These networks help businesses reach a larger audience by placing their banner ads on various websites, often based on user demographics and interests. Advertisers pay the network to display their ads, while website owners earn revenue for hosting them. These networks use technology to track ad performance, such as clicks and impressions, allowing advertisers to measure the effectiveness of their campaigns. Popular examples of banner ad networks include Google AdSense and AdRoll, which facilitate the buying and selling of ad space across the internet.