The optically pumped terahertz (THz) molecular laser was first demonstrated in 1970 by Ta-Yuan Chang and Thomas J. Bridges at Bell Labs. It has since been optimised and refined for the delivery of high-power, coherent THz radiation over a wide range of frequencies and is a workhorse of THz research laboratories around the World. The laser is known by several names in the scientific community, with the most common being the optically pumped far-infrared (OPFIR) laser. It is also known as an optically pumped terahertz laser (OPTL), a THz gas laser and a THz molecular laser. This note provides an introduction to their applications and technology.