The Google Cardboard SDK is an updated version of the Google Cardboard -- made possible by the introduction of the device companion app's new software development kit -- that offers some significant improvements and refinements over existing iterations of the contraption. The biggest highlight of this new version is the fact that Cardboard can now be used in conjunction with Google Street View.
With this new software development kit, the special algorithms that integrate signals from the gyroscope and accelerometer are integrated. This means a reduction in the amount of drift, which is an effect wherein the displayed images continue to move even when you're not turning your head anymore.
The Google Cardboard SDK is also fully compatible with the Unity programming language, allowing the app to work on both iOS and Android metal rendering systems.
However, this update really is about Google Street View first and foremost. The ability to explore distant locales by simply sliding a cardboard box onto your face is really quite spectacular.