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, for example). Mazut is almost exclusively manufactured in the Russian Federation, Kazakhstan, Azerbaijan, and Turkmenistan. This product is typically used for larger boilers in producing steam since the BTU content is high. The most important consideration (not the only consideration) when grading this fuel is the sulfur content, which can mostly be affected by the source feedstock. For shipment purposes, this product is considered a ”dirty oil” product, and because viscosity drastically affect whether it is able to be pumped, shipping has unique requirements. Mazut is much like Number 6 Oil, and is part of the products left over after gasoline and lighter components are evaporated from the crude oil.