"Immersive Virtual Reality (iVR) is the science and technology required for a user to feel present, viaperceptive, cognitive and functional immersion andinteraction, in a generated environment."
For a more detailed definition, see here.
They key concept for Immersive VR is "Presence". Not only cognitive presence (as with books or in the cinema), but also perceptive presence, which naturally fools your senses.
This website is not about SecondLife, WoW or any kind of online virtual worlds.
VR is the experience, it's what happens in your head. The hardware is one part, the software is another part, and you have to have both (and both right) to create the experience !
VR is really used to save time, money, treat phobias, research human perception, protoype cars, marketing research, training.
For more infos, check out this page.