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
