The term VR HMD is basically an abbreviation and means “Virtual Reality Head Mounted Display” and is used for the VR Headsets that use an external piece of Hardware to show content to the person wearing the headset. The hardware would, for this moment, typically be a PC or a VR ready Laptop.
This type of VR Goggles is pretty much the “opposite” of a Mobile VR goggles where you only need a decent Smartphone to play your VR Experience.
These Headsets follow the basic principles of any other VR headset, image is shown to a user where the left and right eye each get served their own image.
Packed with various sensors (like Gyroscope, Accelerometer, Magnetometer) and state of the art techniques (Constellation Tracking, Lighthouse Tracking) the headset is able to track head movements (headtracking) and follow your physical person around the room.
There is even a VR headset that is capable of tracking your eyes ultimately enabling you to really interact with your virtual surrounding.
It is to be expected that, due to the nature of the hardware needed and the retailprice of the first generation VR HMD’s, this will not be the VR Goggles that is up for mass adoption in the short term.