I found a website (not going to share it) that has a lot of really cute backgrounds that you can use on your blog. These are free but you get an ad stuck in the top left corner of your blog. Well, you do if you can't read and understand the code. I edited it out.
I looked around and spotted some code to change the font of my blog on that site. I thought it looked too long and complicated, so I copied it to a plain text reader and looked at it. Woo-hoo! Right after the code to change the font was code that would cause you to send a message to a counting website and raise his 'hit count'. That hit count is how he gets paid for the ads that he has on his own site. If you didn't know any better, you'd copy and paste the whole thing, instead of just the font change.
By including the code on your site, you are allowing hits on your site to be counted as if they were hits on HIS site. So he is getting paid a lot more than he should be. Opening your blog to read it counts as a hit.
Is this legal? Probably not. If the ad company that is paying him based on the number of hits on his website knew that he was boosting his hit count in this manner, he'd lose a lot of bucks. He might even be taken to court. A lot of the hits that he is getting paid for are hits where no ad is showing.
There are also instructions on how to make your own background. Those really are free and there is no extra code included. I'll try them out and post my own version, someday.