The co-op ad network, coded by Shawn Hogan, is a free network of webmasters who display eachothers ads on their websites. Without any investments other than time, this enables the internet marketeer to get his/her webpages and products advertised on literally thousands of websites (hundreds of thousands of webpages) world-wide.
You make adspace on your sites available to others.
You put your ads on adspace of others.
It's coded by Shawn Hogan; it's all automatic (pretty much)!
Just like with Google AdWords, you can have your ads displayed in banner form or text only. It's up to the publisher (similar to AdSense) how to display your ads and how to fit it into their website design.
The Co-op Network Ads have the additional benefit of being plain HTML links. This means you might enjoy increased link popularity as a result of your contribution to the network.