Currently Duo app is listed on Google play store, but showing pre-registration. Means users can't download the app right now, but can register for the app when it will available.
Duo is designed to be a simple 1-on-1 video chat client without all the complications and baggage of Hangouts. It's based on your phone number, so you can chat with anyone in your contact list that has Duo—it's compatible with both Android and iOS out of the gate. It's just one tap to start a call. When someone hits you up on Duo, you'll also see a live video of them before you answer; like a digital peephole.