Jujutsu refers to all skills and forms of sorcery that sorcerers and cursed spirits can access through the manipulation of their own cursed energy. The term "Jujutsu" is an umbrella that encompasses all curse-related mystical arts. It is also often used as a prefix for the vast majority of terms within the jujutsu society. (i.e. Jujutsu Sorcerer, Jujutsu High, etc.)
A person capable of utilizing jujutsu is colloquially referred to as a "Jujutsu Sorcerer" (呪術師 Jujutsushi?) or simply "Sorcerer" (術師 Jutsushi?) while those who can not use jujutsu are called "Non-Sorcerers" (非術師 Hijutsushi?).
It is unknown for how long exactly, but jujutsu has existed for thousands of years, for as long as curses have roamed the Earth.
The Heian Era, the era in which Ryomen Sukuna was defeated is considered the "Golden Age of Jujutsu". Sukuna has described jujutsu sorcerers to have been a problem for cursed spirits in every era. In a never-ending war between sides utilizing jujutsu, a society of organized humans with the ability to fight curses has formed.
In the modern era, sorcerer families that have existed for ages currently control the landscape of their community. The Big Three sorcerer families descend from the Big Three Vengeful Spirits, all of whom used to be famous sorcerers of the Heian Era. Sorcerer families have maintained their prominence thanks to their advanced cursed techniques which are passed down through their family. Due to this, most of the clans are obsessed with their bloodlines and passing down these techniques.
Jujutsu sorcerers currently are selected and trained through the two jujutsu educational facilities in Tokyo and Kyoto. Both schools are considered to be the pillars of the jujutsu community. As the population increases in Japan, so does the strength of curses. The only thing keeping them in check are the crown jewels of the Gojo family: the Six Eyes and the Limitless technique.
Satoru Gojo is the first person in one hundred years to be born with the Six Eyes and the Limitless, making him a near unstoppable force. As cursed spirits increased so did the prominence of curse users, who used their abilities to curse others and exploit them for power. With Gojo's birth, the balance of the jujutsu world shifted the advantage back to the sorcerers dedicated to protecting humanity.
Jujutsu can also dictate fate by binding destinies together. The Six Eyes is connected to Tengen and the Star Plasma Vessel by this type of "fate". Regardless of any outside interference, the Star Plasma Vessel and the Six Eyes would always appear on the day of Tengen's merger. Toji Fushiguro, an anomaly that had broken away from jujutsu completely, was able to shatter their destinies. This was the begging of what would become a massive distortion in the jujutsu world.
Sukuna has been incarnated in Yuji Itadori. Kenjaku was able to acquire Prison Realm and form an alliance with powerful unregistered cursed spirits to seal Satoru Gojo. He's attempting to find what goes beyond the limits of the connection between humans and cursed energy. Using the Culling Game, Kenjaku plans to evolve humanity into something beyond jujutsu sorcerers and cursed spirits. The Culling Game is jujutsu terrorism on a scale never seen before in history, and Kenjaku is using it to usher in the return of the Golden Age of Jujutsu.