from Wikipedia: A resonator is a device or system that exhibits resonance or resonant behavior. That is, it naturally oscillates with greater amplitude at some frequencies, called resonant frequencies, than at other frequencies. Musical instruments use acoustic resonators that produce sound waves of specific tones.
and more from thre electronic side of things:
The oscillator is an active circuit which produces periodic voltage or current signals. The resonator is a passive device whose impedance changes rapidly with frequency over a very narrow frequency range.