Fuel oil is a fraction obtained from petroleum distillation, either as a distillate or a residues from crude-oil distillation. It is used primarily for steam boilers in power plants, aboard ships, and in industrial plants. Commercial fuel oils usually are blended with other petroleum fractions to produce the desired viscosity and flash point.

The term Fuel Oil is the heaviest commercial fuel that can be obtained from crude oil, which heavier than other refined Products. Fuel oils may be generally classified into two main types: distillate fuel oils and residual fuel oils. Distillate fuel oils are vaporized and condensed during a distillation process and thus have a definite boiling range and do not contain high-boiling constituents.

Residual fuel oils contain residues from crude distillation of thermal cracking, and are generally more complex in composition and impurities than distillate fuel oils.

Fuel Oil is also known as heavy oil, marine fuel or furnace oil. In general terms, fuel oil is any liquid fuel that is burned in a furnace or boiler for the generation of heat or used in an engine for the generation of power, Fuel oil is made of long hydrocarbon chains, particularly alkanes, cycloalkanes, and aromatics.