Apple Watch OS is the operating system designed specifically for the Apple Watch wearable device, with features that take advantage of the smaller screen and location of the device on the user's wrist.
Apple Watch OS is based on Apple's iOS operating system for its iPhones and iPads, sharing many of the same capabilities. The Apple Watch OS home screen, or Carousel, arranges icons in a circle on the screen, and from there, users can access Watch-specific features including: Friends, a feature that compiles the user's top contacts; Glances, which provides succinct summaries of news or information from third-party apps users; and advanced timekeeping features such as Time Travel, which shows users upcoming and past events.
Apple Watch OS also includes features from iOS, such as iOS 9's Force Touch capability that allows users to perform different actions depending on how much pressure they when touching the screen. Siri is integrated directly into the OS so users can ask her to start a workout for them or activate Glances, for example.
Apple Watch OS offers a lot of other customization options. For example, users can pick personal or time lapse photos for their watch face, and personalize what shows up on their home screen. This information, also known as complications, can include weather forecasts, the user's schedule, data from third-party apps and more. The OS also allows users to send messages to only other Apple Watch users, including a tap, the user's heartbeat or a sketch from the Digital Touch feature.
For developers, the WatchKit offers a host of application program interfaces to help them create apps with functions specifically designed for the wearable nature of Apple Watch OS.
Apple Watch OS 2, released on Sept. 21, 2015, added new features such as the ability to reply to emails. All the apps also run natively on the Apple Watch OS instead of through an iPhone.