An "ad network" is a network or company that brings together advertisers and publishers. This is done by an ad network offering and selling free advertising space from publishers, on which advertising is to be placed, to suitable advertisers.
Although there are ad networks for TV advertising and print media, an ad network is primarily understood to be a network for online media. The advertising spaces in the Ad Network include e-mails, blogs, streams, websites or RSS feeds.
Ad networks are particularly important in search engine optimization (SEO) and therefore play a major role in online marketing. The most well-known ad network is Google AdWords. Also, ad networks are an element in search engine advertising (SEA), which influences the placement of pages in search engines.