How to distinguish oscillators,resonators and crystals?

[mouthigh]

I am trying to figure out the difference between crystals, oscillators, and resonators. I'm starting to grasp it but I still have some questions.

From my understanding, an oscillator is built from a crystal and two capacitors. What is a resonator then? Is it a difference in terminology?
If an oscillator and a resonator are similar, why do these two items(take them as an example):
HWZT-16.00MD VS  FCR16.0M2G have two pins out and no ground.
Whereas ZTT-16.00MX  has three pins one of which is a ground?

Will any of these three devices work as an external clock for a microcontroller?