It turns out that Google Cardboard is a cardboard ''viewer'' that combines with an app on your smartphone to create a kind of virtual reality headset. Cool stuff. You can access a wide range of Google's apps like Earth, YouTube and PhotoSpheres in an immersive and fun way. If you enjoy the experience enough you can even develop your own demos using the VR Toolkit from Google.
The idea, as cool as it is simple, is directed at one of Google's stated aims to bring virtual reality to as many people as possible without exorbitant costs. Following the developer focus of Google I/O, the viewer tool is meant to help make developing VR apps a much easier experience. All you need is a few simple tools like velcro, magnets, scissors, a rubber band and of course, some cardboard. Then, just download the cardboard design files (direct download from Google) and grab the Google Cardboard app.