Addition reaction is a reaction in which two or more molecules combine together to form a single molecule. This type of reaction occurs only with those molecules which have either a double or a triple bond between them from where these molecules can break one bonds and can add other atoms by sharing those electrons with other atoms(which were previously bonded to carbon atoms).
Thus, addition reaction makes an unsaturated compound into a saturated compound.
CH2=CH2 + H2 → CH3-CH3
Substitution reaction is reaction in which one atom of any other element substitutes or replaces a certain atom or group of a given hydrocarbon.
It takes place only in saturated hydrocarbons(containing only single bonds).
CH4 + Cl2 → CH3Cl + HCl