Elements combine to form compounds, compounds on the other hand also combine with another compound to form a different compounds, these are just some of the reactions that occur in our environment and inside the laboratory.  All these reactions can either absorb or release heat, and therefore has the ∆H.

A particular important process used for tabulating thermochemical data is the formation of compound from its constituent elements.  The enthalpy change associated with this process is called enthalpy of formation (or heat of formation) and is symbolized as ∆H_f, where the subscript f indicates that the substance is formed from its elements.

The value of any enthalpy change is dependent on the conditions of  temperature , pressure, and state of the reactants and products.  That is why , in order to compare the enthalpies of different reactions, a standard  condition is set which is called the standard state.  The standard state of a substance is set at 1 atm pressure, and the temperature is at 25^oC (298 K). And therefore, the standard enthalpy of reaction is defined as the enthalpy change when all reactants and products are in their standard states, and is denoted as ∆H^o, where the superscript "^o" denotes standard-state condition.

The standard enthalpy of formation of a compound is the change in enthalpy for the reaction that forms 1 mol of the compound from its elements, with all substances in their standard states.  By convention, the standard enthalpy of formation of most stable form of any element is zero, because there is no formation reaction needed when the element is already in the standard state. The standard enthalpy of formation is symbolized as ∆H^o_f.  Example is the O₂, molecular oxygen is more stable form than any other form of oxygen O, O₃, and therefore the ∆H^o_f = 0.

Knowing the  standard enthalpy of formation can help us calculate the standard enthalpy of reaction, ∆H^o_rxn.  It can be calculated by the formula below: