Mathematically work is equal to the magnitude of the force times the distance travelled. Heat is a form of energy. Heat is capable of doing work. Heat flows from hotter body to a colder body. Temperature is a measure of the direction of heat flow.
Energy is measured in calories, or joules.
The energy of the universe is constant. A substance is considered to be a fuel, if it produces energy. The most common energy producing reaction is the combustion reaction. The amount of heat produced can be measured, in an instrument called a calorimeter. In an exothermic reaction, the reactants are in a higher energy state than are the products.