In endothermic reactions, the products have more energy than the reactants. Heat is absorbed from the surroundings (e.g., a cold pack).
In exothermic reactions, the products have less energy than the reactants. Heat is released to the surroundings (e.g., a heat pack).
Both reactions require some activation energy to begin the reaction.
Read the following section of Chemistry for Dummies, and write a summary of it: http://docs.google.com/Doc?docid=0ARNbGsVH1evCZGQ5MjluM2dfNzRxam13cXJuZw&hl=en , and make a diagram of an exothermic and an endothermic reaction.