Some guy decided to create a ray-tracing 3D application. He published pictures, video and source code on his website. It took him less than 48 hours to program this software.
Ray tracing is a technique to model the path taken by light by following rays of light as they interact with optical surfaces.
Ray-tracing works sort of like a camera in real life, but in reverse. With a camera (or your eyes, for that matter), rays of light from the environment enter the lens and hit the film/digital chip/meaty eye cells. Something magical happens where the light hits and we get an image!
With ray tracing, we start at each point on our "film" or image, and blast a ray out of our camera lens and see what it hits. What it hits determines the color and brightness at that spot on the film. Of course by "film" I mean digital image, and by "spot" I mean pixel.
I decided my entire program would be centered around one function, called raytrace(). The idea is this: You give raytrace() a starting point and a direction, and it follows that ray until it collides with something in my virtual environment. It returns the color of the object it collided with.