OSVR or Open Source Virtual Reality is a software platform that allows different types of VR technologies to reside in the same ecosystem. This means an end to end compatibility across different brands and devices, allowing you to use any OSVR supported HMDs, controllers and other technologies together, giving you the ability to customize the combination of hardware you would like to use. It is like a software that allows you to customize your VR in the same way you can customize your PC.
There are over 320 (and counting, as of May 2016) OSVR official supporters including companies like Intel, NVIDIA, Acer, Ubisoft, Gearbox Software and Legendary Studios. There are also VR dedicated partners like Leap Motion, Nod Labs, Perception Neuron, Jaunt and the like.