When talking about fire, class defines what kind of fuel is burning. It’s necessary to know which class the fire is to know how best to fight and suppress it.
A Class A fire is the most common fuel, ordinary combustibles such as paper, wood, cloth, or plastic. As you may imagine, the best way to fight these fires is to douse them with water or smothering it with foam, such as from a fire extinguisher.
A Class B fire burns from flammable liquids, such as gasoline or propane, among others. A fire involving cooking oils, while technically a Class B, may also be called a Class K(or Class F in Europe and Australia). For these fires, you don’t use water to extinguish them. Water can actually spread the fuel, causing even more damage. Foam is the best way to suppress a Class B fire. In the case of a kitchen fire, baking soda is actually one of the most effective means of putting it out.
A Class C is an electrical fire. Water and foam cannot be used to combat this type of fire. That could cause the firefighter to be electrocuted. If the power supply cannot be cut, turning it essentially into a Class A fire, special carbon dioxide fire extinguishers need to be used.
A Class D is a fire involving metals. These can include: sodium, titanium, magnesium, uranium, and even calcium. Water cannot be used to fight these fires either. It will actually energize these flames. A dry powder extinguisher is the best method to suppress them. These shouldn’t be confused with extinguishers using dry chemical agents, which can actually aggravate the flames.
As you can see, it’s important to know what is fueling the fire so you know how to put it out. As using the wrong method for that class of fire can actually make things worse.