The iPad Air 2 marks the beginning of a new model number pattern, A15XX. In this case, the iPad Air 2 Wi-Fi bears the model number A1566.
In addition to a new model number, the iPad Air 2 introduces Touch ID into the iPad family.
If you're new to the game, Touch ID allows you to use your fingerprint to unlock your iPad. You can also use it to authorize purchases from the iTunes Store, App Store, and iBooks Store.