3D rendering is the 3D computer graphics process of automatically converting 3D wire frame models into 2D images with 3D photorealistic effects or non-photorealistic rendering on a computer.
Rendering is the final process of creating the actual 2D image or animation from the prepared scene.
Several different, and often specialized, rendering methods have been developed.
These range from the distinctly non-realistic wireframe rendering through polygon-based rendering, to more advanced techniques such as: scanline rendering, ray tracing, or radiosity.
Rendering may take from fractions of a second to days for a single image/frame.
In general, different methods are better suited for either photo-realistic rendering, or real-time rendering.
Non-real time rendering enables the leveraging of limited processing power in order to obtain higher image quality.
Rendering times for individual frames may vary from a few seconds to several days for complex scenes.
Rendered frames are stored on a hard disk then can be transferred to other media such as motion picture film or optical disk.
These frames are then displayed sequentially at high frame rates, typically 24, 25, or 30 frames per second, to achieve the illusion of movement.
When the goal is photo-realism, techniques such as ray tracing or radiosity are employed.
Techniques have been developed for the purpose of simulating other naturally occurring effects, such as the interaction of light with various forms of matter.
Examples of such techniques include particle systems, volumetric sampling, caustics, and subsurface scattering.
The rendering process is computationally expensive, given the complex variety of physical processes being simulated.
(text from wikipedia.com)