Everybody says that a major thing to help with game performance is the hiding of polygons that are not currently seen on screen. How is this accomplished?
How is largely dependent upon the needs of the game. Popular methods for rooms are bsp with pvs augmentation, portals, and oct-trees. Landscapes often use quadtrees, or more complex techniques like view dependent meshes and ROAM.
If the polys are not visible, do not send send them down the graphics pipeline. Clip them first. Read Mike Abrash, etc.