A lot of technology companies like Bizo, Audience Science are helping advertisers target the valuable user segments. These companies have access to huge databases containing information about these users. This information can be used to target B2B users for example – industry segment, functional area, size of companies.
Information to these companies comes from registration information at various places over the internet. Based on the cookie tracking, these users can then later be found & targeted with the ads on other sites throughout the internet like Yahoo!, MSN etc.
It does seem like a simple ad network but with what I will like to stress is that these networks have the information about these users. Of course, this information might not be a 100% accurate but it is still better than using an ad network to target users with no information except their behavior a.k.a. BT (behavior targeting).