The sum of the kinetic energy and the potential energy of a substance is known as its enthalpy. During any reaction there is a difference between the sum of the enthalpy of the reactants and that of the products. If the enthalpy of the products is greater than the enthalpy of the reactants, the reaction is known as endothermic. On the other hand if the enthalpy of the products is less than the enthalpy of the reactants the reaction is called exothermic. Endothermic reactions absorb energy from the environment when they take place while exothermic reactions release energy.
A couple of examples of exothermic reactions are: burning of a fuel, freezing of water, etc. Formation of carbohydrates during photosynthesis, evaporation of water, etc. are examples of endothermic reactions.
A catalyst is substance that changes the rate of a reaction when it is added with the reactants. Catalysts can either increase or decrease the rate of the reaction. At the end of the reaction when the reactants have been converted to the products the catalyst is found in the same state that it was added.