difference between resonator and oscillator

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.

... and a resonator is, usually, the core of the feedback network for an oscillator...

What flatulent answered is very clear.Resonator is just as a LC network , is a part of the oscillator .

Resonator is device which contain energy of alternating electromagnetic field , the best keeping is at resonant frequency and depend from losses in resonator.
Oscillator in general is device , which produce alternating electromagnetic field and use constant field as source of energy and use resonator for the keeping of this alternating field energy.
So , not only LC is resonator, two mirrors in laser are resonator as well.

Resonant almost=oscillator
But,resonant is a natural oscillator

I think Yummy is talking about 2 or three terminal resonators vs xtals as mcu clock sources?. If that is the case, the essential diifference is that resonators are cheaper and the 3 terminal types have built in caps which means they take up less board space. The downside to this is that they are not as stable as xtals, so if you need high stabilty or high baud rates then go for xtal, otherwise resonators are fine. I hope this is what you meant, only guessing.

A resonator can be used to filter signals in a specific frequency range.

A resonator = an oscilator (but for an ideal resonator; in practice, in a resonator, the oscilating signal damps and goes to zero by time)

It is not compulsory a resonator to be passive. It may be active as well. For example if you can not implement an L value in practice, then you can implement it by active circuits.

Resonator is the frequency selective component of an oscillator, and is placed in the feedback path.

In plain language, resonator starts to generate frequency at slight stimulus of voltage/current in proper phase and the active component amplifies these 'oscillations' and ensures that the 'oscillations' are maintained.

There are some good books on oscillator design by Artech house, where one can clarify all such things.
:)

Yes exactly resonant is part of an oscillator