Android TV is Google's second attempt to bring Android to TVs (the first one was Google TV). After Chromecast's success and the launch of Nexus Player, Google partnered with a few TV manufacturers that will launch Smart TVs powered by Android TV.
"This spring, Sony, Sharp and TP Vision, with its range of Philips TVs, will start shipping televisions powered by Android TV. These have all the goodness of Android TV built in so you won't need to plug anything extra into your television. You can use a single remote to watch live TV channels and play games, movies or shows from Google Play and your apps. These televisions feature voice search, to help you quickly find what you want, and they're Google Cast Ready so you can cast your favorite entertainment from your phone or tablet to the big screen," mentions Google.
From The Verge: "Live at CES 2015, Sony's CEO Kaz Hirai announced that the company's new smart TVs will run on Google's new platform for television, Android TV. The remote control for the TV is essentially a giant trackpad, so that you can navigate the Android TV interface more easily. The remote also comes with a microphone, to make searching easier."
Other manufacturers use different operating systems: Samsung uses Tizen, LG uses WebOS, while Panasonic launches Firefox OS Smart TVs. It's interesting to notice that Samsung and LG use their own operating systems to power smartwatches, smart TVs, smart washing machine, smart vacuum cleaners, cars and more. "The OS of everything - Tizen is the open-source operating system for all device areas," mentions Tizen's homepage. Having their own operating system allows them to control their own destiny and launch new devices without relying on software from Google.
