Genjutsu differed from ninjutsu and taijutsu. It targeted an enemy's five senses, plunging them into illusions. Skilled genjutsu users could even manipulate the flow of chakra within an enemy's brain nerves at will. It was a combat method requiring high intelligence and battle sense.
Genjutsu was one of a ninja's three fundamental skills, and many ninja could use it. However, most used genjutsu as support and ninjutsu as their main discipline. Only a handful specialized in genjutsu, such as Itachi Uchiha, Shisui Uchiha, Kurenai Yuhi, Tayuya, Hozuki Gengetsu, Yakumo Kurama, and others.
Genjutsu generally used the five senses as its medium. To evade it, one needed to shut off those senses. Might Guy's unique combat style of not looking into an opponent's eyes could only deal with visual genjutsu. Yakushi Kabuto could block his vision with a snake's eye membrane, but that too only countered visual genjutsu.
According to the official databook, Book of the Sha, there were two ways to break genjutsu: 1. Non-dōjutsu genjutsu: one could disrupt one's own chakra flow to break it; 2. Dōjutsu genjutsu: one could only break it by having another person disrupt one's chakra flow. In addition, perfect Jinchuriki who had reached an understanding with their tailed beasts could resist genjutsu attacks.
Sleep paralysis genjutsu was a relatively unusual category of genjutsu. Known examples included the Uchiha Clan's "Demonic Illusion: Shackling Stakes Technique," Jiraiya's "Sage Art: Toad Confrontation Chant," and Tayuya's "Demonic Flute: Phantom Sound Chains." Its defining trait was that the victim knew they had fallen under genjutsu, yet their body could not move. Note: sleep paralysis referred to the condition in which a person was mentally awake while sleeping but unable to move their body, commonly known as being "pressed down by a ghost." A "sleep paralysis technique" was a paralysis