@temi: Co-op is an ad network that runs throughout over 20.000 websites. You have to place a php code at every page of a website, via an algorithm there are points given to your website, with this points (weight) it determens how many times your ads are shown in the network.
In the admin you can create ads (textads and banners) these ads show up in the network and are spidered and improve your rankings. You can see an example at http://www.whatifind.com/ (footer) where my links from the network are shown.