With the release of iOS 6, Apple replaced its controversial unique device identifier system (or UDID) with the Advertising Identifier. It's a non-permanent, non-personal, device identifier, that advertising networks use to track your behavior.
Since its introduction, users who don't like targeted advertisements have had the ability to limit ad tracking via a toggle in the Settings application. And in iOS 6.1, users will gain the ability to reset their Advertising Identifier as well...