In an exothermic reaction the enthalpy of the products is less than the reactants, hence you feel the heat. In an endothermic reaction the reverse is true, since the products have more enthalpy than the reactants. This occurs at the expense of its surroundings. Specifically it can occur without the introduction of external energy. A nonspontaneous reaction cannot occur on its own.
Spontaneous reactions occur when the free energy of the system decreases.
During nonspontaneous reactions the free energy of the system increases. Based on free energy movement in a system the terms exergonic and endergonic are used to determine the direction of this free energy. An exergonic reaction will release energy from the reaction, while an endergonic reaction will absorb free energy from its surroundings. In order for an endergonic reaction to go to completion it needs an outside source of energy. ATP helps the cell carry on 3 main types of work.