This document discusses the basic principles and characteristics of lasers. It defines lasers as devices that produce coherent beams of light through stimulated emission of radiation. The key concepts covered include absorption, spontaneous emission, stimulated emission, population inversion, and Einstein's coefficients. Lasers achieve population inversion through pumping mechanisms like optical pumping or electric discharge. Their beams exhibit high directionality, intensity, monochromacity, coherence, and a narrow divergence angle.