3d objects are perceived to us as signals from both of our eyes, both are giving 2d images of the world, but the brain makes it 3d by blending together the points of view. If you look hard enough, you will find that computers and many other things use projections to their advantage
You can project a 3d object onto a wall quite easily, such as a semi-transparent cube, so long as the sides can be distinguished, with colour. All you have to do is to find an object and point a torch at it with a wall behind, your result will be a silhouette of the object from the torches point of view, and a semi-transparent object will produce a marvelous projection of that cube, and at the right angle, it will look as if the cubes shadow is 3d!
3d in Computers
Computers are very powerful nowadays, and they have worked out many ways to project the mess of 1’s and 0’s into a 3d object on a 2d screen, they do it with the same method that the shadow projections do. The difference of the shadow projections and the computers projections is that it makes the viewpoint that of the camera and the shadows are displayed. There are other methods too, such as warping objects to produce a 3d effect and even making a 3d scene and using ray tracing to figure out the 3d effects.
3d in Pictures
In pictures, many artists have gotten the idea of image projection and have used that to produce images that look quite cool, and some have even made images that fool us!
They do this by memorizing the rules of 3d, being shadows, size, foreshortening, overlapping, surface lines and density, and a couple of the other unwritten ones too.
A digital projector uses an extremely bright bulb to project something onto the wall, such as in a cinema, they are the source of the 3d effects and many of the computing methods use the idea of a projector to their advantage.