Online advertising provides a revenue stream for website owners and a promotion and marketing channel for advertisers. BuiltWith can track online advertisers via there banner signatures embedded into a page. Most advertisers provides two channels, one for their advertising partners and one for their web site owning customers. There are many different web advertising companies, some very large such as Google provide text and image advertising and have huge markets, other smaller companies provide more niche advertising management.
Google Adsense is the most used advertising system found on BuiltWith scanned sites, DoubleClick, the second most tracked technology, was finally approved for purchase (by the FTC and EU) by Google in 2008 making Google control the majority of the non-search specific internet advertising.