Online advertising is a popular revenue model where websites earn money by hosting ads from advertisers. Websites join ad networks that connect them with advertisers. Advertisers pay for ad placements using pricing models like cost per click (CPC) or cost per thousand impressions (CPM). Common ad formats are banners, videos and search ads. Publishers must meet the ad network's requirements for content and traffic. This model provides benefits like tracking performance but also risks like ad blocking and click fraud.