The letter indicates the type of bulb, the number is diamater in 1/8" increments. Additional letters may indicate type of filament, frosting, base type, distance from base to filament center, etc.
A few types off the top of my head:
A - standard "light bulb" style G - Globe PS - Pear shaped T - Tubular PAR - Parabolic Aluminized Reflector R - Reflector MR - Mirrored Reflector
