A behavioral ad network is a network which collects web users’ behavioral data for the purpose of advertising targeting.
The biggest behavioral networks can encompass hundreds or thousands of websites participating in data collection. Google’s Ad Sense which is the biggest network with behavioral tracking and displaying capabilities counts more than two million partner websites.
For fall 2012, Audience Science network claimed 2,7 billion pixel hits per day (registered events), 270 billion of active registered facts in database and a reach of 220 million users.
Behavioral tracking done on Yahoo Network:
Retargeting network are also a particular form of behavioral ad network.
For more details on behavioral networks operating principles, see behavioral targeting.