Types of ad networks

There are 3 main types of online advertising networks:
Vertical Networks: They represent the publications in their portfolio, with full transparency for the advertiser about where their ads will run. They typically promote high quality traffic at market prices and are heavily used by brand marketers. The economic model is generally revenue share. Vertical Networks offer ROS (Run-Of-Site) advertising across specific Channels (example: Auto or Travel) or they offer site-wide advertising options, in which case they operate in a similar fashion to Publisher Representation firms.
Blind Networks: These companies offer good pricing to direct marketers in exchange for those marketers relinquishing control over where their ads will run, though some networks offer a "site opt out" method. The network usually runs campaigns as RON or Run-Of-Network. Blind networks achieve their low pricing through large bulk buys of typically remnant inventory combined with conversion optimization and ad targeting technology.
Targeted Networks: Sometimes called “next generation” or “2.0” ad networks, these focus on specific targeting technologies such as behavioral or contextual, that have been built into an Ad server. Targeted networks specialize in using consumer clickstream data to enhance the value of the inventory they purchase. further specialized targeted networks include social graph technologies which attempt to enhance the value of inventory using connections in social networks.
There are two types of advertising networks: first-tier and second-tier networks. First-tier advertising networks have a large number of their own advertisers and publishers, they have high quality traffic, and they serve ads and traffic to second-tier networks. Examples of first-tier networks include the major search engines. Second-tier advertising networks may have some of their own advertisers and publishers, but their main source of revenue comes from syndicating ads from other advertising networks.
While it is common for websites to be categorized into tiers, these can be misleading. While Google is in the clear majority of advertisement impression served, other networks that could be labeled as tier 2 actually dominate over these tier 1 ad networks as far as the number of customers reached.