A decomposition reaction is the one in which a compound decomposes into two or more than two substances (elements or compounds).
For example, lime stone when heated gives lime and carbon dioxide.
CaCO3 (s) + heat → CaO (s) + CO2 (g)
In the reactions in which decomposition occurs by application of heat, such reactions fall in the category of thermal decomposition. For example,
2Pb(NO3)2 (s) + heat → 2PbO (s) + 4NO2 (g) + O2 (g)
A reaction in which a compound decomposes due to electrical energy into two or more than two substances (elements or compounds) is called electrolytic decomposition reaction.