Fuel oil is a type of petroleum product that is used primarily for heating and powering engines. It is derived from crude oil through a refining process and is heavier than gasoline. Fuel oil is commonly used in residential heating systems, industrial boilers, and ships, providing a reliable source of energy. There are different grades of fuel oil, including No. 1, No. 2, and No. 6, each with varying properties and applications. No. 2 fuel oil is the most widely used for home heating, while No. 6 is often used in large industrial applications due to its higher viscosity and energy content.