Crystal and Oscillator
The crystal is one component of a crystal oscillator. The oscillator is basically an inverting amplifier with the crystal in the feedback path. The crystal's task is to let just one frequency pass through.

In a microcontroller the oscillator's amplifier is built in the IC, and the feedback path for the crystal is available at two pins: input and output.

In itself a crystal can't do anything. It's a passive component.