An "R" lamp is a common reflector, simply directing light in one direction. A "BR" is a bulged reflector, able to direct a little more light in the central part of the beam. "PAR" stands for parabolic aluminized reflector", which is a reflector with an interior reflecting surface able to project a narrower beam than an "R" lamp. The numeric value represents the diameter in eighths of an inch. A "PAR38", for example, has a diameter of 38 eighths of an inch.