Webdude : Very simply, CoRegistration is sharing one set of details with more than one company / site.
i.e. I signup to a newsletter. They have a few checkboxes beneath the signup form saying "do you want to sign up for xxx newsletter, yyy daily ezine" - if the user clicks yes, their details are sent and I get a small referral fee.
you get cash, they get a potential lead.
heres an example :
http://www.joke-around.com/pic/100 (disclaimer - thats my site. its not adult. there is advertising. you may see...gasp...a popunder) - look at the tell a friend form at the bottom. it has the little check boxes I mentioned earlier.