Virtual Reality (VR) is the use of computer technology to create a simulated environment. Unlike traditional user interfaces, VR places the user inside an experience. Instead of viewing a screen in front of them, users are immersed and able to interact with 3D worlds.
By simulating as many senses as possible, with the focus on vision & hearing (touch & even smell can be embeded as well), the computer is transformed into a gatekeeper to this artificial world.
VR is consumed through an virtual reality headset, which is a head-mounted device that provides virtual reality for the wearer.
They comprise a stereoscopic head-mounted display (providing separate images for each eye), stereo sound, and head motion tracking sensors (which may include gyroscopes, accelerometers, magnetometers, structured light systems etc.
Some VR headsets also have eye tracking sensors but most have some kind of controller.