An IDFA is an identifier for advertisers that is used on iPhones instead of cookies. It allows advertisers to understand when a user takes an action like clicking an ad or installing an app. Unlike cookies that expire quickly, an IDFA does not change unless a user changes it, so it can provide a persistent identifier to build consumer profiles. The IDFA gives users more choice than the previous UDID identifier by allowing them to opt out of IDFA tracking or change their IDFA.