HMDs are the most popular form of VR equipment. They are goggles that connect to a computer system, and the computer does the the graphics processing. As such, you are not as mobile, but depending on the setup, you may still have enough mobility for the most immersive VR experience of them all.
The project that revived the dream of VR, the Oculus Rift, is considered an HMD. Since the Oculus Rift, however, there have been many other companies announcing their own versions. Among them are the HTC Vive or otherwise known as SteamVR, and Sony’s PlayStaion VR, the recently renamed Project Morpheus. There’s also Razer’s OSVR, or Open Source Virtual Reality, which will be incorporating Intel RealSense tech. This probably means gesture controls instead of having wired gloves or something along those lines for a less tethered VR experience.