Rami Arieli: "The Laser Adventure" Chapter 3 page 2
Every oscillator has 4 main parts (as seen in figure 3.1): 1. Amplifier.
2. Positive resonance feedback.
3. Output coupler.
4. Power source.
Figure 3.1: Electronic Oscillator

In analogy to the electronic amplifier, the laser can be described as composed of four structural units (see figure 3.2):

 
Figure 3.2: The Basic Laser System