what is rendering????
A rendering is a photo realistic view of your CAD model. Most programs have an modul to create a realistic screenshot. There also exit specialiced programs like keyshot which "only" create renderings of several CAD solutions like solidworks, CATIA, proE andd may more.
source: see also: http://en.wikipedia.org/wiki/Rendering_(computer_graphics)
Rendering is the process of generating an image from a model (or models in what collectively could be called a scene file), by means of computer programs. A scene file contains objects in a strictly defined language or data structure; it would contain geometry, viewpoint, texture, lighting, and shading information as a description of the virtual scene. The data contained in the scene file is then passed to a rendering program to be processed and output to a digital image or raster graphics image file. The term "rendering" may be by analogy with an "artist's rendering" of a scene.