The Samsung Gear S runs on Tizen, which is Samsung's homemade operating system. Samsung has other smartwatches that run on the OS, but also models that run on Google's Android Wear. Samsung markets Tizen as having a growing ecosystem and has been working on releasing the open source OS on a smartphone. The drawback of having Tizen's ecosystem is that it is not as robust as Android's or Apple's, but Samsung says the watch will feature 1,000 apps.
Apple Watch runs on a modified version of iOS that is specifically built for the smartwatch. The company stresses the point that it is not simply a condensed version of their mobile operating system, and it is tailored for the smaller screen. The advantage of being an iOS-based platform is the ecosystem of 1.3 million apps. Apple released the WatchKit SDK to developers yesterday in order to have a slew of watch-specific applications ready to go upon release.