What is retargeting?
Someone visits your website. We place a retargeting pixel on certain sections of your site or across your entire site. As visitors to your site visit the pages you have the pixel on they are added to what is called an "audience." We drop a cookie on this audience member. As they visit other sites on the web they are shown your ads. The larger this qualified group of past visitors, the more effective your retargeting is.
If a person in your audience converts then a "burn pixel" will initiate, which takes them out of the audience. You can also choose to add different pixels if you wish to show converting visitors different ads. You can get quite complex regarding the sequence of pixels you cookie these visitors with.
You can also control the sites that show your ads, just like on typical display network buys. You can decide that a site isn't cutting it, and cut them from the list. You have quite a bit of control around who sees your ad and who doesn't.
Whether you decide to start out with a smaller audience (maybe you decide to JUST cookie those users that made it a cart page but didn't complete a purchase) or whether you decide to put every possible visitor to your site into the audience, this tracking method is incredibly effective.