File used by Passbook, an iOS 6 app used for saving boarding passes, coupons, loyalty cards, movie tickets, and other passes; may contain a pass provided by a vendor (e.g., Starbucks, Delta Air Lines, etc.) or a custom user-generated pass; can track confirmation codes, logins, balances, and other information.
PKPASS files can be imported into Passbook simply by downloading them with Safari on your iPhone. Vendors that distribute passes must digitally sign them to verify the issuer of the pass.
PKPASS files are actually .ZIP files that contain logo images, pass formatting information, and a digital signature.
NOTE: An Android app called PassWallet can also open and manage PKPASS files.