Mazut is a heavy, low quality fuel oil, used in generating plants and similar applications. In the United States and Western Europe, mazut is blended or broken down with the end product being diesel.

Mazut may be used for heating houses in former USSR and in countries of Far East that do not have the facilities to blend or break it down into more traditional petro-chemicals. In the west, furnaces that burn mazut are commonly called "waste oil" heaters or "waste oil" furnaces.

Mazut-100 is a fuel oil that is manufactured to GOST specifications, for example GOST 10585-75 or 99. (GOST is the Russian system of standards, much like ASTM D396, for example). Mazut is almost exclusively manufactured in the Russian Federation, Kazakhstan, Azerbaijan