Free vibration:

When a body capable of vibration is displaced slightly from its equilibrium position and then left to itself, the body begins to vibrate freely in its own natural way called the free or natural vibration of the body with a definite frequency.

Forced vibration:

The oscillations of a body under the influence of a periodic driving force (when the two periods differ) are called forced or damped vibration.

Resonance: If an object or source of sound is under the action of a periodic force and frequency of the periodic force is same as that of the source, it will vibrate with the large amplitude. The phenomenon of producing sound of large amplitude is known as resonance.