Ad networks are sophisticated operations designed to allow advertisers to place their advertising materials in front of selected individuals. The hope (and often the promise) is that these individuals are good prospects for the particular product or service on offer. But ad networks differ markedly in several ways, including:

• How these individuals are selected or targeted
• The range of publishers' sites on which the advertisements may appear

A network can also be categorized according to the nature of the financial arrangement between the advertiser, the network and the publisher. It may be a true network based on revenue sharing agreements across a wide range of sites, an arbitrage network that buys unused or unwanted inventory at bargain prices in hopes of repackaging and reselling it profitably, or a broker network that simply manages the transactions and adds no value for advertisers.

Despite the differences of emphasis, organization and approach, most networks offer a range of options, including demographic, geographic and dayparting. Many allow advertisers to mix and match various types of selectivity, creating a near infinite range of possibilities for finding specific categories of prospects and serving them targeted messages.

To help you cut through the complexity, here are brief descriptions of the major characteristics of today's most effective ad networks, each one accompanied by a list of ad networks that fit the descriptions.

Note: Many of the Networks we list in the following pages offer a range of services and therefore appear in multiple appropriate categories.