When we deal with exothermic processes, it is supposed that such chemical reactions generate heat (or energy). Almost all combination reactions are exothermic (for example, 2Na+O2=Na2O2). If you count up its enthalpy (H), it is going to be negative (to count it up you should find in your handbook standard enthalpies of reagents and products (ir there is a single type of atoms like Na or O2 - their enthalpy is always zero. Then sum up the whole enthalpy of reagents and the same for products individually. Then from the enthalpy of products take the enthalpy of reagents. This value shows the energy effect of a reaction, when it is negative, the reaction is exothermic). Using enthalpy it is possible to predict whether reaction is exothermic or endothermic.
Endothermic reactions have positive enthalpy value. When the reaction is endothermic, it absorbs heat. Almost all decomposition reactions are endothermic (e.g. AgNO3 = Ag + NO2 + O2). Endothermic reactions have positive enthalpy value.