Basically run games anywhere with a chrome browser at up to 4k with HDR at launch.
We saw bits and pieces of this with the Project Yeti test earlier this year.
This includes a new controller, but this will work with the controllers that already exists.
Stadia uses the Vulkan API, so that will probably mean some substantial effort written with DX.
Each session will take place on a 10 TFlop instance. This is more powerful than current consoles, but it sounds like you need specialized api's to actually run on Stadia's instances.
Stadia should launch in 2019. We will learn more about launch titles in the Summer.
No pricing has been announced.