A decomposition reaction consists of a single reactant that breaks down into two or more simpler products.

Example: Decomposition of calcium carbonate to give calcium oxide and carbon dioxide as,

\(CaCO_3+Energy \rightarrow CaO + CO_2\)

A combination reaction consists of two reactants that combine or mix together to form a single
product.

Example: Combination of Calcium oxide and carbon dioxide to form one single product calcium carbonate

\(CaO+CO_2 \rightarrow CaCO_3+energy\)

Hence, decomposition reactions are the opposite of combination reactions.