Decomposition reactions are those in which a compound breaks down to form two or more elements or new compounds.
These reactions require a source of energy (heat, light or electrical) to proceed. Thus, they are called the exact opposite of combination reactions, in which two or more reactants combine to give a new product with the release of energy.
For example:
Decomposition reaction: CaCO3(s)Heat→CaO (s)+CO2(g)
Combination reaction: CaO(s)+H2O(l)→Ca(OH)2(aq)