Genjutsu (meaning "Illusionary Technique") are techniques that are employed in the same fashion as ninjutsu, requiring chakra and hand signs. However, the primary difference bewteen the two is that the effects of genjutsu are illusionary; instead of attacking the victim's body, genjutsu techniques manipulate the flow of chakra in the victim's brain, thus causing a disruption in their senses.
Genjutsu is created when a ninja extends their chakra flow through the cerebral nervous system of their opponent to control their mind's chakra, thereby affecting their five senses. Those with special abilities like the Uchiha Clan's Sharingan or with high intelligence have an easier time executing and countering genjutsu, as attention to detail is key. Those under the influence of genjutsu either freeze in place or lose subconsciousness, depending on how capable they are in recognizing and depending against it.