First letter will stand for pad shape type (obviously you will need to check for specific pad letter to match your caliper). Basically most popular for nowadays will be:
G, F, J (Deore-XTR) - narrow pad shape
B (Acera-Altus) - wide pad shape (you can not use these with narrow/2piece rotors)
D, H (Saint-Zee) - narrow pad shape
K, L (for Road 140mm rotors) - narrow pad shape
A, E, M (older calipers) - vary from model
Two numbers will define pad compound
01-02 - Resin (except D02S which is Metallic, maybe some other older marking as well like E01S)
03-04 - Metallic
05-08 - vary from model (older calipers)
Last letter will define backplate material
A - Aluminum
C - Combined (Aluminum fins, Steel pad backplate)
S - Steel
Ti - Titanium