watchOS is the operating system that runs on the Apple Watch. The Apple Watch shipped with watchOS 1.0 — roughly analogous to iOS 8.2 — in April of 2015, but was updated to watchOS 1.0.1 — roughly analogous to iOS 8.3 — in May of the same year.
Apple announced watchOS 2 as part of WWDC 2015 on June 8. It includes the return of the Photos and Time Lapse clock faces first seen at the Apple Watch introduction back in September of 2014. There's also third-party Complications for App Store apps, Time Travel to see data from the past and future, a Nightstand mode for when you're charging at bedside, the ability to reply to emails, sketch in multi-color, and to have groups of Friends. Siri is smarter. Maps got transit directions, Apple Pay got store and loyalty cards. And yes, there are full-on native apps.
Codenamed after beaches, watchOS uses the same backboard and frontboard systems as the iPhone and iPad, but with Carousel instead of Springboard as the app launcher and manager layer, and with a watch face system uniquely its own.
On September 21, 2015, watchOS 2 was released to the public, offering a slew of new features including photo and time-lapse clock faces, glimpsing backwards and forwards in time, responding to mail, adding more friends, direct networking, workouts on lock screen, and much more. Read all you need to know in our full watchOS 2 review.