Forced Vibrations:
If an external periodic force is applied to a freely vibrating body, the body tries to maintain its vibrations at its own frequency, but after some time, the body begins to vibrate with the frequency of the applied periodic force. Such vibration of a body is called a forced vibration.
Resonance:
When the frequency of the applied periodic force matches the natural frequency of a vibrating body, its amplitude rapidly increases to a large value. This phenomenon is called resonance.