For example take this snapshot on the right. Some clever programmer made software that surfs the internet filling out forms with website addresses. (yeah, that's a snapshot of the "ad". If this doesn't entice you to visit "drugs-kingdom.info" about your inner ear infection I don't know what will.)
Note: This is similar to another "bot" that simply visits your website and leaves its "card" in your traffic logs. They make it look like visitors came to your site from a link on the one they are advertising. The curious website owner will visit the other site and find that it's just an ad.