With the introduction of iOS 6, Apple also rolled out a new method of tracking users for advertisers and developers: IFA, IDFA or "ID for advertisers". Although the technology cannot be used to personally identify a user, it does however offer a detailed look into the user's browsing habits and ostensible interests. Business Insider notes though, the feature may be curiously tricky to disable for some users who wish to disable it.
IFA aims to fill the gap left by Apple's ban on developers using UUIDs to target ads. Unlike IFAs, which are temporary, randomly generated numbers capable of being destroyed and re-issued, UUIDs are unique strings which permanently identify a device -- forever. UUIDs also have the distinct disadvantage (or advantage, depending on your point of view) of being linked to your real-world identity, as this massive UUID leak showed us most recently.
Disabling the feature is simple enough -- but you have to know where to look. For whatever reason, Apple tucked away IFA's options in Settings under General > About > Advertising, instead of just putting it under Privacy, oddly enough. To further obfuscate matters, the actual ad control is called Limit Ad Tracking which needs to be enabled in order to disable tracking. By the way, shame on Apple for forcing users pay attention and read.
IFA is expected to be a boon for advertisers. The feature works a bit like a cookie, following users around when they visit websites and use certain apps. If a user has been shopping around for "ultrabooks" recently, advertisers can pick up on that and offer ultrabook ads to that user. Unlike previous methods, IFA also provides conversion, allowing ads to link directly to an app or purchase so companies can better monitor the effectiveness of their ad campaigns.