"Very well, sensei. Genjutsu is are performed in the same fashion as that of Ninjutsu; through hand seals and chakra. Genjutsu literally means illusionary techniques, and as the name suggests, these attacks target the mind, not the physical body. These attacks disrupt the senses, which many times renders the victims body useless. It is categorized as a form of Yin release.
Genjutsu can be used to create fake images within the mind of the user, as well as fake pain, by targeting the senses to make them believe they are under stress to the point of causing pain. They do not, however, actually harm the victim's physical body, so they aren't user quite as frequently as ninjutsu techniques. Genjustu can also be used to, in a sense, hypnotize a person, giving one the ability to produce suggestive images in the mind of the victim, giving you control over them. When an interrogation must occur, genjutsu is a great way to break ones will and find out the truth.
By controlling the chakra flow among the cerebral nervous system and affecting the five senses, genjutsu is created. It can cause the victims body to appear unconscious, and can also allow the victim to remain in motion, allowing them to think they are doing something that may not be totally accurate; for example- the victim sees my face and throws a punch, when it is actually the blade of my sword.
Genjustu, more times than not, is used by ninja with high intelligence and very good chakra control. If it isn't done correctly and precisely, it simply will not work.
Any species with a prosencephalon structure of the brain, such as dogs and humans, can fall victim to genjutsu. Some species, such as insects, lack that structure, and rely soley on their senses, making them completely immune to genjutsu. This means that an Aburame clan user would be a good counter to a genjutsu user.
Genjutsu can occur through different ways, such as sound and sight. They can even be associated with elemental jutsu, however this is less common.
To overcome genjutsu, the is more than one method. The less pretty method would be to cause immense pain to oneself, which brings the senses back to their original order. This seems to be the most effective method, and is an easy way to break genjutsu, but takes a toll on your body.
The method that sounds best is to stop the flow of chakra through your body, and then apply an even stronger power to disrupt the flow of chakra of the opponent that casted it. This method is known as Genjutsu Dissipation, and can even be done by an outside force by applying a surge of chakra into the affected person(s). A form of this would be how Killer Bee and the Hachibi work together and break genjutsu through their cooperation in a similar fashion.
Another method is based on the clan you were born into... so this only applies to the Uchiha Clan or outside members with a Sharingan. The Sharingan can see through genjutsu. It also allows them to literally go inside of the genjutsu that is affecting a comrade and break it from there.
There are multiple ways to break a genjutsu, however some, such as Itachi's Tsukuyomi, can't be broken out of so easily."