Global illumination is a very popular method for lighting three dimensional scenes. Unfortunately, most of the methods are very slow and requires lots of ressources. Ambient Occlusion is a method for simulating global illumination, while keeping a respectable ratio between results and computing times.

As well as anything related to raytracing with Blender, because of the supplementary computing time required by your computer, raytracing is only an option that you should feel free to activate and deactivate. This is done by the mean of the Scene menu (F10 key). The Render tab shows a button labeled Ray you will have to activate in order to use raytracing in your pictures.

goto Tutorial :

http://feeblemind.tuxfamily.org/dotclear/index.php/2005/01/16/10-didacticiel-locclusion-ambiante—tutorial-ambient-occlusion