I found this while looking around the net recently. It’s a group of advertisers that set cookies on your computer when you visit certain sites, then based on your surfing habits will present targeted advertising direct to your browser. If you find this kinda creepy, here is a single page that will allow you to opt out of these companies setting the cookies
hit ‘select all’
This will cause all of the cookies set by these companies to be reset to not include data from your browser, and verify the status. You will probably find that many of these have a big red X next to them on the next page (particularly if you use Safari) because Safari blocks these by default. I decided to try it out and set my browser to ‘accept all cookies’ then tried to opt out of all of the available advertising networks. Only one didn’t seem to take, a company called ‘Red Aril’. I’ve sent an email to the company to find out why, and unsurprisingly did not receive a reply.
If you don’t like seeing ads it may also be worthwhile to download and install Safari AdBlock
It’s also available for Chrome, Firefox etc.