An advertising network is a company that connects advertisers to web sites that host advertisements. The key function of an ad network is an aggregation of ad space supply from publishers and matching it with advertiser demand. The phrase 'ad network' by itself is media-neutral in the sense that there can be a 'television ad network' or a 'print ad network', but is increasingly used to mean 'online ad network' as the effect of aggregation of publisher ad space and sale to advertisers is most commonly seen in the online space.
The fundamental difference between traditional media ad networks and online ad networks is that online ad networks use a central ad server to deliver advertisements to consumers, which enables targeting, tracking and reporting of impressions in ways not possible with analog media alternatives. A desktop ad network caters to advertisers on applications for desktop computers and webpages.