The laser is a system that is similar to an electronic oscillator.
An Oscillator is a system that produces oscillations without an external driving mechanism.
To demonstrate an oscillator, we can use the familiar acoustic analog:
A sound amplification system has a microphone, amplifier and speaker.
The whistle is created spontaneously, without any external source.
The speaker's internal noise is detected by the microphone, amplified and
the amplified signal is again collected by the microphone. This positive
feedback continues until a loud whistle is heard.