A-paper, wood, cardboard, and most plastics
B-flammable or combustible liquids such as gasoline, kerosene, grease and oil
C-electrical equipment, such as appliances, wiring, circuit breakers and outlets
D-chemical stuff (like a lab) They are for fires that involve combustible metals, such as magnesium, titanium, potassium and sodium.
I believe there is a K too - involve cooking oils, trans-fats, or fats in cooking appliances and are typically found in restaurant and cafeteria kitchens.