The Identifier for Advertisers (short: IDFA) is a temporary device identifier used by most Apple devices. It provides user information to advertisers so they can serve relevant ads to their users on websites and mobile apps.
The IDFA replaced the Unique Device Identifier (UDID) which was used prior to iOS6. But in contrast to UDID, it does not identify the device or user. Instead it groups different users by similar search or browsing behavior for relevant and targeted advertising.
The identifier is non-permanent and can be activated or deactivated by the user. Additionally, users now have the possibility to reset their IDFA, similar to how desktop users can erase cookies from their browser.
The IDFA allows advertisers to target certain users who have already been active in the past. Therefore, it is commonly used by ad networks, retargeting providers, and other media sources to serve targeted ads and re-engage users on specific devices.