In the case of film speed, the higher the number, the more light sensitive it is. 

The F-Stop does indeed affect DOF in real world cameras, not sure about Luxrender.  The bigger the number, the smaller the aperture.  (Not really, it is really reversed but most cameras drop the '1/'xx' from the display.)  The smaller the aperture, the more of the scene is in focus.  However, the smaller the aperture, the less light is used.