"One of the first tasks that firefighters and inspectors learn is how to classify fires in accordance with the common letter designations. Knowing the correct fire type enables one to select the appropriate fire-extinguishing media to safely suppress a fire. Fires are classified into one of five categories based on the burning products. The following table explains the classes and the description of the primary contents involved in the fire."
Coffee Break Training - Fire Protection Series No. FP-2014-41
United States Fire Administration: http://www.usfa.fema.gov/