As stated on Wikipedia : "Google Cardboard is a virtual reality (VR) platform developed by Google for use with a head mount for a smartphone. Named for its fold-out cardboard viewer, the platform is intended as a low-cost system to encourage interest and development in VR applications. Users can either build their own viewer from simple, low-cost components using specifications published by Google, or purchase a pre-manufactured one. The viewer is used by placing a smartphone into the back of it and viewing through the lenses in the front."
It works on both Android and iPhone.
You can find out more about Google Cardboard by visiting this link: https://www.google.com/get/cardboard/