Tizen is an open source software platform from Samsung which is designed for use on multiple platforms. Some devices which utilize the Tizen OS include smartphones, smart watches, tablets, in-vehicle infotainment devices, smart TVs, and more. Tizen is considered an alternative to the Android OS and is a successor to the Bada OS (also from Samsung).
The Tizen SDK allows you to develop HTML5 applications for Tizen devices and runs on Windows, Ubuntu and Mac OSX.
Samsung's Galaxy Gear Fit and Gear 2 both run on the Tizen platform.