Apple Pay requires an Apple device with near field communication (NFC) capability. So, Apple Pay will be available on the new iPhone 6 and 6 plus for in-store and in-app purchases.  Apple Pay is also available for the new iPad Air 2 and iPad Mini 3, for in-app purchases only.
Owners of the iPhone 5, 5c or 5s will have to purchase an Apple Watch to utilize the technology since it requires the NFC chip.