It should be noted that virtually all renderers do frustum culling, meaning anything in the game world that is guaranteed to be out the camera's viewing volume is not rendered. Frustum culling is quite simple. Occlusion culling is usually done after frustum culling and is more complex, and the method tends to vary depending on the scene type and game engine, or is sometimes not done at all (modern GPUs are so powerful that smaller or simpler game areas render fast enough without occlusion culling).
