Ray tracing is a widespread technique in computer graphics. You’ve probably seen stills and movies that involved ray tracing, whether in the graphics community or in movie theaters or on TV. The distinguishing feature of ray tracing is that it is one of the most photorealistic ways of rendering computer images. A trade off when using ray tracing is an increase in rendering time, so ray tracing is not used as often in real time situations.

Subsequently, the traditional way of rendering in some 3D programs and most games is not raytracing, but a process called “rasterization” that attempts to do the same thing, but much faster, though slightly less accurately. Ray tracing’s advantage over rasterization is that it’s capable of many more phenomena by default than rasterization, due to its property of being modeled on the real world. It’s this property, and the basic process of raytracing, that will be discussed in this article.

http://www.pxleyes.com/blog/2010/03/5-things-you-need-to-know-about-raytracing/