An advertising network (also called an online advertising network or ad network) is a collection of (often unrelated) online advertising inventory.
Online advertising inventory comes in many different forms. This inventory can be found on websites, in instant messaging applications, in adware, in e-mails, and on other sources. Some examples of advertising inventory include: banner ads, rich media, text links, and e-mails. (This is not an exhaustive list.)
Large publishers often sell only their remnant inventory through ad networks. While not commonly known, even among many large publishers remnant inventory can exceed 50% of total inventory, although this is not always the case. Typical numbers range from 10% to 60% of total inventory being remnant and sold through advertising networks.
Smaller publishers often sell all of their inventory through ad networks. One type of ad network, know as the blind network, is such that advertisers place ads, but do not know the exact places where their ads are being placed.
In most cases, ad networks deliver their content through the use of a central ad server.
Large ad networks include a mixture of search engines, media companies, and technology vendors.