Fuel oil is a fraction obtained from petroleum distillation, either as a distillate or a residue. Broadly speaking fuel oil is any liquid petroleum product that is burned in a furnace or boiler for the generation of heat or used in an engine for the generation of power, except oils having a flash point of approximately 40 °C (104 °F) and oils burned in cotton or wool-wick burners.

In this sense,  diesel is a type of fuel oil. Fuel oil is made of long hydrocarbon chains, particularly alkanes, cycloalkanes and aromatics.

The term fuel oil is also used in a stricter sense to refer only to the heaviest commercial fuel that can be obtained from crude oil i.e. heavier than gasoline and naphtha.

Number 1 fuel oil is a volatile distillate oil intended for vaporizing pot-type burners. It is the kerosene refinery cut that boils off right after the heavy naphtha cut used for gasoline.

Number 2 fuel oil is a distillate home heating oil. Trucks and some cars use similar diesel fuel with a cetane number limit describing the ignition quality of the fuel.

Number 3 fuel oil was a distillate oil for burners requiring low-viscosity fuel. ASTM merged this grade into the number 2 specification, and the term has been rarely used since the mid-20th century

Number 4 fuel oil is commercial heating oil for burner installations not equipped with preheaters.It may be obtained from the heavy gas oil cut

Number 5 fuel oil is a residual-type industrial heating oil requiring preheating to 170 – 220 °F (77 – 104 °C) for proper atomization at the burners. This fuel is sometimes known as Bunker B.

Number 6 fuel oil is a high-viscosity residual oil requiring preheating to 220 – 260 °F (104 – 127 °C). Residual means the material remaining after the more valuable cuts of crude oil have boiled off.

Distillate fuel oil sales totaled 57.0 billion gallons in 2012. This decrease of 61.7 million gallons is the smallest year-on-year change since data were first collected. It is also the first annual change of less than 100 million gallons. Though decreasing, this represents a stable market as distillate fuel oil’s share of total sales surpassed the 90 percent mark for the first time.

The 2012 total is nearly 6.2 billion gallons below peak levels achieved in 2007. Decreases can be seen in the residential, commercial, electric utility, military, and transportation sectors. Bucking the trend, sales to the industrial and oil company sectors grew nearly 6.7 and almost 23.9 percent, respectively.