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It may not be your high end gaming VRE headset, but with just a few pieces of cardboard and a phone (Nexus 5 should be but you can adapt), you'll get a fully working VR headset with audio, gyroscope, 3D, etc. That little metal ring on the side is actually a trigger (using the Nexus 5 magnetic sensor).
Google Cardboard is more of a standard than a single product. It started with a cardboard box with glass lenses and a slot to hold your phone, but now there are many different hardware solutions that are all just as capable as the next.
Specs
Pros
Cons
Con Very basic
Google Cardboard was Google's way of generating interest for phone powered VR, and is literally just a piece of cardboard (or plastic) and glass lenses. The software is integrated into the Google Play Store, and Google more-or-less put Cardboard out there to see what developers would do with it. Later in 2016, Google will be making their next push forward with Daydream which will be a more modern, well thought through platform.
Con The official box is for Nexus 5 only
Even though you may apply the technique to other phones, the prebuilt one is only working well with Nexus 5 because of the screen size, NFC sensor, camera position, and magnetic sensor. However all of these are optional and it'll work as long as your phone screen size is roughly 5 inches.
[Additional Information:] Many phones can fit with minimal modifications to the Cardboard.