Apparently, iOS 6 introduced IDFA, "identifier for advertisers", which identifies your device so that advertisers can track you and send you ads. It appears they also introduced IDFV, "identifier for vendor".
How do IDFA and IDFV work? What exactly do they identify? Are they different for each app on your phone, or are they the same for all apps on your phone?
What are the privacy implications of IDFA and IDFV? Can they be used to track you? How do they compare to UDID and to cookies on the web, as far as the privacy impact?
Do users have any way to tell which apps are gathering this information? Also, if you set "Limit Ad Tracking" to On in settings, what happens under the covers? How does that change what information apps receive?