In order for developers and marketers to track your usage of your iOS devices, most of them relies on a unique identifier of your device (Device UUID). Apple provided API to access this device UUID until iOS 5. Since then, developers are advised to make use of a different API using the so called Advertising Identifier in order to identify and track your device.
iOS 6 makes it easy to disable the use of this Advertising Identifier. Go to Settings > About > Advertising and turn on the “Limit Ad Tracking” switch.
Please note that it takes time for all tracking apps to convert to use this new Advertising Identifier as Apple has yet to ban the use of Device UUID. API for Device UUID is still in iOS 6 and will only be removed in future iteration of iOS. Furthermore, some tracking apps might use its own tracking method instead of relying on this Advertising Identifier.