Murim is a term used for The Martial Arts World while Murim-in refers to the people who are a part of said world.
No one knows when Murim first came into existence in the world. However, what’s definite is that Murim possess a different strength and mentality compared to normal people. In order to stop the possibility of the normal world and the martial arts world colliding together, for generations murim have created Dojo's and federations to keep the appropriate distance between murim and normal people. Because of this Murim-ins and society have been able to coexist for a very long time. However, people like Goomoonryong broke those ancient rules. They infiltrate the normal world while having murim-in powers. It is these breakers that threaten the balance.
The murim is a world maintained by the passage of martial arts from the previous generation to the next. Accordingly, the relationship between master and disciple must be more intimate and earnest than a blood relationship between father and son.
Soon after their appearance, these monks became famous throughout the society as martial arts masters because of their powers. In response, many people quickly met them in order to become their disciples. In time, with more masters and disciples appearing, the amount of murim increased, clans and schools were formed and the world of Murim was born. Due to the nature of the murim being "not normal", since their strength is far above the ordinary, there was the fear that coexistence between normal people and murim would create conflict and, in time, war. Thus, the existence of the Murim world was kept secret from normal people, with the exception of some members of local governments and federal government. The Murim is governed by the Martial Arts Alliance. The federation seems to be in conflict with a second group, known as the Black Forest Defence who wishes the destruction of the first for a reason not revealed.
There are rules the Murim-in must follow, when a battle between Murim-in they must inform Martial Art Alliance of the battle. And if a Murim-in tries to take a life then they have to be prepared to risk there own life as well.
Although unknown when the term was first used, it now signifies the whole of the martial arts practitioner in the world. However, its origin be traced through its meaning, forest. Apparently, a group of ascendents tried to achieve enlightenment through mediation in a forest. However, they created martial arts in order to defend themselves against dangerous animals of the forest.
Since the strength between humans and beasts defers greatly the amount of strength needed by humans was also great. Thus normal martial arts practiced by most people were not enough. Although martial artists (especially in Eastern countries) know about the existance of Ki the only ones to realize its true potential were the monks that practiced in the forest. Instead of using the Ki for simple concentration and better mastery of the body's function these monks find the way to directly use the Ki for Movement, Attack and Defence. The boost given in this new usage of Ki made people capable of running faster than cars (Walking Techniques), climb to the roof of building by jumping from wall to wall (Wall Tiger Skill), break walls or cut steel platforms with kicks or punches (Soul-Crushing Strike, Focus Stomp, Split Heaven Kick) and also boosted in a great extent the martial arts styles that are also know to normal people (for instance, Master Yewon of the Yewon-Gwan School is able to perform normal throwing moves that are able to send people flying several meters to the air thanks to the use of Ki). Thus, the martial arts of the Murim world were created.