The nx will be a console and you can use you're mobile phone to play the game on the go. The controller is a plastic thing that you can put your mobile in,and connect it with Bluetooth. It works like the vita/ps4 connection, but with good controls
It only works with WiFi and you're phone is the display, so even old phones work with it.
Probably down the line they make their own screen with even better controls I think.
When you're at home you have controller and the console itself. (again just like ps4 vita connection but this time it works better because you'll have all the buttons)
The Nintendo app is only downloable for Apple and Android users, not Windows users for some reason.