For now, let’s talk 3D.
When something is made for 3D, you’re essentially seeing two cameras. In a 3D movie, that’s a camera rig setup for the positioning between a left and a right eye, while computer animated movies and post-processed movies into 3D (and there are a lot of these) rely on computer-based 3D rigs.
These rigs emulate the position our eyes take, which is why each lens is technically seeing a different part of a 3D move: the lens on the left sees more of the left, while the lens on the right sees more of the right, sort of like what happens when you take in the scene in front of you — anywhere really — and close one eye at a time.
Close your left eye and you see a little more on the right. Close your right eye, and you see a little more on the left. Leave both eyes open and you take in the entire scene.
That’s 3D, and the depth behind 3D is explained by distance. The further the distance, the less pronounced the depth, but the closer it comes to you, the more an object is reaching to you, though this can be engineered in films and video games (and often is).
Virtual reality is like 3D, but a little different again.
Imagine a scene where you’re seeing the entire world around you, not just a set of pre-recorded images that move in front of you.
A 3D movie is a pre-recorded movie set up as one specific point of view, usually controlled by a director, be it from a movie or a game.
In virtual reality, however, the scene you see isn’t necessarily set, and you can shift your position by moving your head.
Turn your left from left to right and the view will change, realigning your view with that of the world, often with the depth from 3D.
VR is therefore like 3D, but wider and more open. It often encompasses the depth needed for 3D, but doesn’t have to.