Wikipedia
watchOS is the mobile operating system of the Apple Watch, developed by Apple Inc. It is based on the iOS operating system and has many similar features. It was released on April 24, 2015, along with the Apple Watch, the only device that runs watchOS. Its API is called WatchKit.
The second version, watchOS 2, included support for native third-party apps and other improvements. It was previewed publicly at WWDC 2015 in June 2015, and released on September 21, 2015, delayed seven days by a bug that took longer than expected to fix.
The third version, watchOS 3, was released in beta to developers at WWDC 2016 in June 2016, and is slated for public release in fall 2016. It emphasizes better performance and includes new watch faces and stock apps, including Reminders, Breath, and Home.