Global illumination refers to a class of algorithms used in 3D computer graphics which, when determining the light falling on a surface, takes into account not only the light which has taken a path directly from a light source (local illumination), but also light which has undergone reflection from other surfaces in the world.
Images rendered using global illumination algorithms are often considered to be more photorealistic than images rendered using local illumination algorithms. However, they are also much slower and more computationally expensive to create as well. A common approach is to compute the global illumination of a scene and store that information with the geometry. That stored data can then be used to generate images from different viewpoints for generating walkthroughs of a scene.
Radiosity, ray tracing, cone tracing and photon mapping are examples of global illumination algorithms.
Also monte-carlo or distributed ray tracing as well as variations like the metropolis light transport algorithm model diffuse inter-reflection which is a very important part of global illumination.
