What is real time Rendering ?

Rendering is the process of converting Geometric description of Objects to picture. To do computations regarding lighting of a scene , it takes a while to finish the process. A Physically based Photorealistic computation will some time takes a day and more for rendering even a frame of graphics. ( you require 24 frames to show one second of a movie ...! ).

People came up with much cheaper lighting calculation which gives good visual fidelity( Phong illumination model [Ambient , Diffuse , Specular etc] was the dominant one till hardware accelerated graphics came. Now , using GPU you can have other lighting models in real time ). This helped to render scenes in milli seconds time per frame. Coupled with hardware acceleration , good interactivity was achieved. if you are rendering a scene at least 16 frame per second , it is called real time rendering. This is essential for Games , CAD and other scientific applications.

The cheaper lighting calculation is called Local illumination model. To have physically correct rendering , we require Global illumination calculation like Radiosity , Photon mapping etc.