To all intents and purposes, the world we live in is three dimensional. Therefore, if we want to construct a realistic computer model of it, the model should be three dimensional as well.
Unfortunately, the usual display device of a computer (the monitor) is 2 dimensional. However, its seems that 2 dimensions may be enough to convince the viewer that he or she is watching a realistic model of the real world. For example, the images seen on a television.
Nevertheless, we have been restricted to two-dimensional television and two-dimensional cinema for many years and seem very happy with it. This is because the images we get on the TV have many subliminal visual cues that tell us "This is 3 Dimensional". And the brain believes what the eyes tell it even though we know the screen is flat! Thus we can conclude that a 2-D screen can do a good job of presenting the 3D world, if we do it right!
We have a not insignificant hurdle in working in 3D on the computer. The screen in flat, it has width and height but we also need depth. Depth is the dimension into and out of the screen. All computer screens are made up from small rectangles of color, each rectangle is called a "pixel" (short for picture element), the more pixels on the screen the higher the resolution of the screen.
It is usual to specify the resolution of the computers display by quoting the number of pixels across the screen and the number of pixels down the screen rather than the very large number of pixels on the screen! For example a low resolution output for a display of 320 pixels across the screen by 200 pixels down the screen requires 64000 pixels in total.
Two dimensional art, animation or drafting (CAD) packages usually use a graphics cursor (an arrow or some other icon) so that you can make a specific action at an appropriate place on the screen. They may also use a co- ordinate reference (rather like a map-reference) to specifically refer to a point on the screen. Like a map reference, you need two numbers to specify a point on the screen, an across number and a down number. Again like a map reference you need a fixed reference point. This is usually taken as the top left corner of the display and is referred to as (0,0). Moving across the screen points are referenced as (1,0) (2,0) etc. Moving down the screen points are referenced as (0,1), (0,2) etc. So any point can be defined with two numbers. For example the center of the screen might be (160,100).
In 3D we need to add a third number to the across and down coordinates (a depth coordinate "in/out") giving us (0,0,0), (0,0,1) etc. We have a problem in visualizing this third coordinate. Because the screen has no depth, we cannot see a move that goes in/out. Ideally we would like a second screen on the side of the monitor to see the depth move.
We can't expect computer manufactures to make monitors with screens on the side as well as the front, even if we (as users) were prepared to move round to the side to look at it!
However, we have designed our 3D software so that it folds this 'imaginary' screen on the side of the monitor round, until it lies in the same plane as the monitor's screen. It is also helpful to fold an imaginary screen from the top of monitor so that we have three views arranged on the screen and all visible at the same time.
Our next difficulty arises because the mouse only works when it is on a flat surface. You cannot pick it up and move it about on the side of a computer monitor! But wait, you don't pick the mouse up and move it about the screen for a 2-D drawing either. You just assume that its pointer is on the screen in front of you.
Therefore, let's assume that when the mouse pointer is in a part of the screen which we are using as one of the folded views. it moves as if it were on the side or top of the monitor.
There we are; that's got the 3D screen and mouse sorted out.