In short, virtual reality (VR) gives users the ability to be immersed in an environment different than the one they currently are standing in. Typically this environment is conveyed through a head-mounted display that ideally shows stereo imagery to the user. As the user moves their head around, various tracking technologies can synchronize changes in the user's head motion with changes in what part of the displayed environment they see. More involved VR systems can add in positional tracking for both the head and hands. Emerging developments include smaller, lighter-weight, and higher fidelity displays, that incorporate new ways of tracking parts of the human body.
Check out this introductory video from Visual Vocal, to get a flavor for how we apply VR to fields such as architecture, construction, and engineering:
If you want to go deeper on understanding what virtual reality is, you may want to check out this excellent publication from the Chaos Group, the company that makes the VRAY renderer:
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