Class A:
Ordinary Combustibles such as wood, cloth and paper.
Class B:
Flammable liquids such as gasoline, oil and oil based paint.
Class C:
Energized electrical equipment- including wiring, fuse boxes, circuit breakers and appliances. 
Class D:
Dry powder extinguisher is suitable for use on metal fires. Tested and approved for fires involving magnesium, sodium (spills and in depth), potassium and sodium potassium alloys.

Class K:
Designated "Class K" under the US system, this extinguisher is suitable for use on Cooking oil or fat.