-Genjutsu basically means illusionary techniques. It is similar to normal ninjutsu except that its effects are illusionary.
-Genjutsu techniques manipulate the flow of chakra in the victim's brain, thus causing a disruption in their senses.This is often used to create false images or to cause pain from trauma (because the body is led to believe it is in pain); however, there are plenty of other uses depending on the situation.
-Genjutsu is created when a ninja extends their chakra flow through the cerebral nervous system of their opponent to control their mind, thereby affecting their five senses.
-Those with special abilities like the Uchiha clan's Sharingan, Kurama clan or with high intelligence have an easier time executing and countering genjutsu, as attention to detail is key.
-Genjutsu attacks usually require a lot of skill, and, if performed incorrectly, will not work; even Jiraiya had trouble performing such techniques. Genjutsu is most often used by ninja with very precise chakra control and very high intelligence, the most notable being Itachi Uchiha and Kurenai Yūhi.
-Because genjutsu affects activity throughout the Prosencephalon structure of the brain, humans, dogs and other species of mammal are therefore all affected by genjutsu; however, the kikaichū, like all insects, lack such a structure to their brains, and so do not even notice it.Some animals have also been seen to use genjutsu;Jiraiya's Ma and Pa
To combat genjutsu, a ninja can:
-The ninja needs to stop the flow of chakra in their body, and then apply an even stronger power to disrupt the flow of the caster's chakra; this is called Genjutsu Dissipation
-Inflict intense pain not caused by the genjutsu and utilising the resulting pain to bring their senses back in order (of the first two options, the latter seems more effective in most situations, or at least is the easiest).
-Sharingan wielders can also use their dōjutsu to liberate any ally caught in an enemy genjutsu by casting their own genjutsu on them, literally breaking into the illusionary world they are imprisoned in order to free them. This can be achieved through eye contact alone.