Watermarking Mp3s/Adding a license

Alright, I don't know exactly what it's called and how they do it.

But I've noticed on Limewire and other programs that some MP3s and videos refer you to a website when you download them. Also: when you view a list of music or video files in thumbnail view they'll all have descriptions of the files under them.

How do you do this?

Check out pay-per-install.org