Those who wish to advertise online can do so on more than one website, through an advertising network that represents numerous websites. In this way, advertisements can be purchased either on the entire network, without exception, or wherever advertisements of a particular category appear on the network.

So as to avoid conflict with their own sales representatives, some advertising networks do not sell advertisements that are site-specific, at all. In any case, advertisements that are specific to any particular site or sites do not receive strong emphasis while sales are being made through advertising networks. Advertisements that run over an entire network or over an entire category, as mentioned before, are what the campaigns often involve.

While smaller networks may accept sites, the page views for which run into a few thousand each month, the larger networks generally accept sites that represent strong brands and the page views per month for which often number in millions.

When the websites are free to display advertisements from sources other than the network that they have an agreement with, the agreement is termed as non-exclusive. On the other hand, in the case of an exclusive agreement, the sites have to display advertisements sourced through the advertising network only. An exclusive agreement usually implies a greater share of revenues being earned by the advertising network than in the case of a non-exclusive agreement.

Money is increasingly being paid, however, by companies that run advertising networks to companies that develop software, as well as those running websites, for having advertisements displayed when users log on to a software application or when they access the websites.

So, advertising networks are essentially agencies acting as mediaries, between the advertisers and those who are willing to display advertisements.

Advertising networks are also often referred to as advertising agents, especially when the Internet is exclusively the environment where the advertisements are being displayed. The revenue is usually shared on the basis of the number of times the advertisements are clicked on, by those visiting the websites that the network has an agreement with. The inventory of advertisements that a network has, can consist of advertisements for services or products that are completely unrelated, at times.

Text links, rich media and banner advertisements, among others, are part of the inventory of advertisements that networks can get displayed, on behalf of advertisers. These are displayed in emails, on websites, in adware, in RSS feeds, in instant messaging applications, on blogs, among other forms of advertising online.

The advertisements reside on the advertising network's server, from where these are accessed, when a user clicks on an advertisement displayed on any of the websites that have an agreement with the network. For example, when a user clicks on a banner advertisement, a code snippet that represents it and is present on the server of the advertising network, is called immediately.

Even as smaller publishers may rely to a larger extent on advertising networks, larger ones generally require their services only to assist with the sale of advertising space that has remained unsold through direct sales or other channels. As a matter of fact, though, many of the smaller publishers sell all of their advertising space through such networks.
Technology vendors, media companies and search engines are among the major advertising networks.

There are many different kinds of advertising networks. The three main types are:

1. Representative networks: Revenue sharing is generally the economic model in this case. These networks tend to be used by established brands and mostly promote traffic of high quality. The advertisers are fully aware of the locations where their advertisements are to be run and the system is more or less transparent.

2. Blind networks: Arbitrage is the economic model in this case. Such networks purchase the inventory of advertisements that remains with publishers and, so, are able to offer low prices to advertisers. However, the advertisers do not control the location of the advertisements, in any way, in this case.

3. Targeted Networks: The data for the stream of consumer clicks are used, in this case, for providing targeted advertising and, therefore, enhancing the value provided to the advertisers. Contextual or behavioral criteria are used for the purpose of focusing the advertisement content, in accordance with consumer preferences.