What is POV-Ray?
POV-Ray is a freeware ray tracer. It is available for most computer platforms in use today, including - Mac OS, DOS, Windows, Unix, Linux, Amiga etc... If your favorite platform doesn't have an official compile, the source code is freely available. You can download it and compile your own executable. You are even free to modify the source to add your own custom features.
POV-Ray is a powerful ray tracer with many features usually available in expensive commercial 3D programs.

Of course there is a bit of a catch to this. Most commercial 3D programs consist of a couple of parts. They have a modeler where you visually create the models that will populate your 3D scene. Then they have one or more renderers that read the scene file and draw the scene to your computer screen.

POV-Ray is a renderer only. It's up to you to supply a model or scene, in the form of a text file which POV-Ray will read and render. POV-Ray uses ray tracing as its rendering algorithm. Here is a description of how ray tracing works from the POV-Ray manual:

"Ray-tracing is a rendering technique that calculates an image of a scene by simulating the way rays of light travel in the real world. However it does its job backwards. In the real world, rays of light are emitted from a light source and illuminate objects. The light reflects off of the the objects or passes through transparent objects. This reflected light hits our eyes or perhaps a camera lens. Because the vast majority of rays never hit an observer, it would take forever to trace a scene.

"Ray-tracing programs like POV-Ray start with their simulated camera and trace rays backwards out into the scene. The user specifies the location of the camera, light sources and objects, as well as the surface texture propereties of objects, their interiors (if transparent) and any atmospheric media such as fog, haze or fire.

"For every pixel in the final image one or more viewing rays are shot from the cmera, into the scene to see if it intersects withany of the objects in the scene.These 'viewing rays' originate from the viewer, represented by the camera, and pass through the viewing window (representing the final image).

"Every time an object is hit, the color of the surface at that point is calculated. For this purpose rays are sent backwards to each light source to determine the amount of light coming from the source. These 'shadow rays' are tested to tell whether the surface point lies in shadow or not. If the surface is reflective or transparent new rays are set up and traced in order to determine the contribution of the reflected and refracted light to the final surface color. " 1

Ray tracing is capable of creating photorealistic images complete with shadows, reflections, transparent objects and realistic light refraction. Because it must calculate a light ray and its interactions with any objects it strikes in the scene, for each pixel in the final rendering, ray tracing can be a time consuming and computationally intensive process.

POV-Ray has its own scene description language, which is similar to a programming language such as 'C'. This is what makes POV-Ray somewhat intimidating to learn. It is not really as difficult to learn as a person may think when first encountering it and with a little effort one can learn it well enough to create some impressive scenes. There are also quite a few utilities and modeling programs available which will let you visually create a model or scene and output it in POV-Ray readable form.

POV-Ray User Manual by the POV-Ray Team