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.
In the world of The Breaker, the term martial art is derived from the term forest. Legend states that martial arts originated with a group of monks as a means to defend themselves from the dangerous animals they encountered while meditating in the forest.
They realized that martial arts (as it had existed at that point) was inadequate when compared to the drastic difference in strength and power between themselves as humans and the animals of the forest. Through experimentation with ki outside of their traditional medicinal practices, these monks discovered they were capable of previously unknown levels of superhuman athletic ability. In the modern age, the practices have given rise to techniques which enable individuals to run 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 and punches (Soul-Crushing Strike, Focus Stomp, Split Heaven Kick) as well as enhance the effectiveness of other martial arts styles practiced by common civilians. Master Yewon of the Yewon-Gwan School, for example, is capable of throwing opponents several meters into the air through the use of Ki to enhance basic throwing techniques.