They're currently called "doubleclick for publishers" or DFP for short – and they're a suite of ad management tools. It's basically an ad server that allows you to serve ads on your site, and then get reporting on impressions/clicks/etc that you can send out to advertisers, or use for your own uses. They have a free version for small businesses, and there is a paid and supported version that is used by some bigger companies.
They're pretty big, and chances are if you have ads on your site, at some point they're being served by DFP.