Exothermic reactions transfer energy to the surrounding environment. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to become hotter. If the products contain less energy than the reactants, heat is released or given out to the surroundings and the change is called an exothermic reaction.
Some examples of exothermic reactions are:
Neutralisation reactions between acids and alkalise
The reaction between water and calcium oxide