Rami Arieli: "The Laser Adventure" Chapter 3.5 page 1
3.5 Operation of a Laser System - Interactive Applets:

In the following set of interactive demos, the basic principles of operation of a laser system are described.

At first, the active medium is not excited, no emission is observed, as can be seen below:

 When the pump energy is turned on, energy flows into the system, but the energy is not enough to create population inversion. In this case only spontaneous emission can be seen. To see the result, Click here for low power pump:

When the pump energy increases, a population inversion occurs, and we can see both spontaneous and stimulated emission. To see the result, Click here for high power pump:

 When mirrors are added, they form an optical cavity around the active medium. The stimulated emission becomes dominant in a specific direction (the optical axis of the laser). Click here to add optical cavity:

When the amount of reflectivity of one of the mirrors is changed, part of the laser radiation is emitted out of the laser cavity. Click here to be able to change the reflectivity of the output mirror: