There are two types of labels: one that uses colored geometrical shapes with the appropriate letter designation, and the other is a picture/symbol labeling system designed to make the selection of extinguisher easier.
Class A extingushers will put out fires in ordinary combustibles, such as wood and paper. The ratings refer to the amount of water they hold, and therefore the amount of fire that they will put out.
Class B extinguishers are suitable for use on flammable liquids, such as grease, gasoline, oil, etc. Ratings refer to the approximate amount of fire that a non-expert operator can extinguish.
Class C extinguishers are suitable for use on electrically energized fires. This rating simply means that the extinguishing agent is non-conductive.
Class D extinguishers are designed for use on flammable metals, and are often specific for the type of metal in question. These extingushers generally have no rating, and are not given a multi-purpose rating for use on other types of fires.
Many extinguishers available today are suitable for use on different types of fires, and so will be labeled A-B, B-C, or A-B-C. Every exingusher of this type must be clearly labeled, and if it is not, insist that the proper label be applied.