When a body capable of vibration is subjected to the action of an external periodic force, the vibrates irregularly at initial stage but later on vibrates with frequency of the external periodic force. This type of periodic vibration is called forced vibration.
Resonance is said to occur when a particular body or a system is set in oscillation at its own natural frequency by an external source which vibrating with the same frequency. So resonance is a special type of forced vibration.
We can conclude from the above definitions that all resonances are forced vibrations but all forced vibrations are not resonances.
