Official Google Blog: Making ads more interesting: "ads are a valuable source of information — one that can connect people to the advertisers offering products, services and ideas that interest them. By making ads more relevant, and improving the connection between advertisers and our users, we can create more value for everyone. Users get more useful ads, and these more relevant ads generate higher returns for advertisers and publishers."
what this also means is now your experience of any given website's advertising will now become different from my experience on the same site; the adverts we'll see will still be driven by that site's content, but now become influenced by our respective surfing behaviours as tracked by the adsense we've seen elsewhere.
on the privacy side, no, Google isn't keeping track of where your cookie has been, but if you've gone on a binge of watching websites on how to separate U232 by whirling a bucket of Uranium slop, be aware that shadows of that interest might be detectable in the Visit Afghanistan! adverts your local MI5 agents will find when they check out your laptop.
to peek at and maybe pro-actively tweak what Google thinks you're like, you can visit your Adsense Manager and edit your cookie.