Section 2.1: Principles of Lasers--What is laser?
Lasers, broadly speaking, are devices that generate or amplify light, just as transistors generate and amplify electronic signals at audio, radio or microwave frequencies. Here light must be understand broadly, since lasers have covered radiation at wavelengths ranging from infrared range to ultraviolet and even soft x-ray range.
In the following section, we will try to make you understand the laser principles more in depth. A laser device is consisted of: (1) laser medium like atoms, molecules, ions or semiconductor crystals; (2) pumping process to excite these atoms (molecules, etc.) into higher quantum-mechanical energy levels; and (3) suitable optical feedback elements that allow the beam of radiation to either pass once through the laser medium (as in laser amplifier) or bounce back and forth repeatedly through the laser medium (as in a laser oscillator). (Extract from the book "Lasers" page 2 by Anthony E. Siegman)
For details of Basic Constructions of Lasers, please refer Level 1 chapter 2 section 2.4