Opting out of something you don't even know exists can be more than tricky, it can be impossible.
"In iOS 6, however, tracking is most definitely back on, and it's more effective than ever, multiple mobile advertising executives familiar with IFA tell us. (Note that Apple doesn't mention IFA in its iOS 6 launch page).
Users can switch off that targeting, but it's tricky, as we discovered a couple of days ago. Although at least iOS 6 users are able to turn off tracking, which they weren't before.
Here's how it works.
IFA or IDFA stands for "identifier for advertisers." It's a random, anonymous number that is assigned to a user and their device. It is temporary and can be blocked, like a cookie.