Please explain clearly and in short about the differences and boundaries between AR, VR & MR.
Virtual Reality: where the user is completely ‘immersed’ in a virtual environment. Nothing of the outside world is visible.
Augmented Reality: information is added to the real world by overlaying graphics over the real world the user sees.
Mixed Reality: a combination of the above where the user sees the real world, but virtual objects are placed in this world (often anchored) as if these were real. These virtual objects have the illusion of being part of the real world and can be interacted with. At a higher level, a virtual lightswitch could for example control real lights: therefore ‘mixed’ reality.
Virtual Reality and Augmented Reality actually exist on a spectrum, so the definition between them can get blurry. The distinction right now is mostly driven by hardware.
The easiest factor to determine the difference today is immersion. How much of the user's experience is virtual content? At one end of the spectrum is an entirely virtual world. At the other end is the physical world.
Microsoft's name for this spectrum is "Mixed Reality".
Where physical space is on one end, containing AR experiences. The other end is virtual space, where VR experiences happen. This is why Microsoft calls their VR headsets "Mixed Reality" headsets, with a further classification of "Immersive devices".
Others will define the gap between AR and VR as MR, and the entire spectrum as XR (either as Extended Reality or where X is a variable replacement for A,V,M):
There's another spectrum to consider as well, spatial understanding. This spectrum adds another dimension to the immersion spectrum. It goes from understanding nothing about the physical environment, like Google Glass or Vive to understanding a lot about the environment, like the HoloLens or Magic Leap.
At the moment, spatial understanding is less important for VR devices, other than for automatically setting boundaries, so the user doesn't walk into a wall or other physical obstacle when in VR. It's much more important for AR devices like HoloLens or Magic Leap, to know how virtual objects should be occluded or where flat surfaces are for dynamic content.
At some point in the future, there will be a device that can span this entire spectrum. But right now we're hardware limited to multiple devices to cover just tiny parts of the entire spectrum.
Accordingly to Peter Patterson (XR Architect / Developer), Unity officially stands by the Consumer Technology Association definition of the terms which read as follows.
Virtual Reality creates a digital environment that replaces the users real-world environment.
Augmented Reality overlays digitally created content into the users real-world environment.
Mixed Reality is an experience that seamlessly blends the users real-world environment and digitally created content where both environments can coexist and interact with each other. For instances, Jaron Lanier defines Pokémon GO as the heart of mixed reality.