What is a exothermic reaction?

An exothermic reaction is one where the products have less energy than the starting reactants.  This means the reaction must give out energy, usually in the form of heat or sound.  As a result of this, H (the enthaply of the reaction) must be negative as the reaction has lost energy.

On the other hand if a reaction takes in energy, then it is known as endothermic.  Here, the products have more energy than the reactants, and H will be postive.