It normally refers to a number of flammable liquid mixtures of hydrocarbons, a component of natural gas condensate or a distillation product from petroleum, coal tar, or peat boiling in a certain range and containing certain hydrocarbons. It is a broad term covering among the lightest and most volatile fractions of the liquid hydrocarbons in petroleum. Naphtha is a colorless to reddish-brown volatile aromatic liquid, very similar to gasoline.
Fuel oil is a fraction obtained from petroleum distillation, either as a distillate or a residue. 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.
Diesel or diesel fuel in general is any fuel used in diesel engines. The most common is a specific fractional distillate of crude oil.