Whats the use in The Objects “Sensor” and “Occluder”
they are both in the release notes on blender.org. http://wiki.blender.org/index.php/Dev:Ref/Release_Notes/2.49/Game_Engine#Physics
Basically sensor objects are for advanced physics stuff and occluder is for optimising rasterizer performance. If you enable occluder then all objects behind will not be sent to the rasterizer, this have to be used carefully to get good improvements from, it’s well described in the link.