The Apple Watch is compatible with the iPhone 5 model and up. This is because the Apple Watch requires supporting devices to run iOS 8 or higher.
The Apple Watch runs on an interface based on iOS. However, it is not a stand-alone device with a fully operational operating system, so users must pair it with an iOS device to make the most of its features. Save for a few native Apple applications, most applications and functions require pairing with an iPhone running a compatible version of iOS. For example, it does not have its own GPS capabilities, so it cannot track distance without an iPhone. It can function as a viewfinder for an iPhone camera, but does not contain its own camera. In addition, it requires iPhone pairing to connect to the Internet through Wi-Fi.
However, it does have a few stand-alone functions. The Passbook application, which stores things like movie and plane tickets, can run without iPhone pairing. It can run the Music application and store music natively on the device. It also runs Apple Pay, Apple Remote and the Health application independently of the iPhone. In addition, it can function as a watch, complete with timers, stopwatches and alarms.