In his article "VR 101," Heim describes immersion as one of the three defining characteristics of virtual reality. Immersion isolates the users senses and makes them a full participant in the virtual world. There are various levels and types of immersion, but it becomes difficult to quantify the particular experiences since they depend on each individual's background and psychology.
Heim discusses three types of stronger tools of virtual reality, as well as three others which are more limited and less immersive. One type is the HMD, or head mounted display. These helmets force the user to shut out their primary world and take all sensory input from the virtual world. A setback of this type of tool is that it can become very uncomfortable for the user. A second category includes networks and simulators, which is often employed for military applications such as SIMNET and flight simulators for pilots. A third type is "the cave," or CAVE automatic virtual environment. This is where graphics are projected on the walls of a small room and the user wears stereo synchronized glasses and uses a navigation wand to manuver. This has been used for scientific visualization and discovery as well as automobile design. The three limited modes of virtual reality are the BOOM, desktop VR and VRML. The only examples of these types of virtual reality experiences that I have encountered are simply going to the IMAX theater, which included special effects as well as the 3-D glasses, or to a place like Main Event, where you can play games which use a HMD type method of virtual reality.