The illusionary arts are known as Genjutsu. Now, Genjutsu use techniques which produce illusions in a targets mind, which effectively allows the user to have full control of the targets five senses since Genjutsu affects all senses. However, unlike Ninjutsu, Genjutsu techniques produce imaginary effects on the target, and not any real physical attacks.
A person is put under the effects of Genjutsu when the user sends chakra to the targets brain, affecting their chakra flow system by altering the brainwaves. This is what causes the target to see illusions. Just like other techniques, Genjutsu also requires the usage of chakra, and most of the techniques also require the performance of hand seals. But a few do not need hand seals to be performed.
So, what is the purpose of putting someone under a Genjutsu? Well, there are a few reasons. One would be to torture the opponent by giving them mental pain and trauma to make that person feel helpless and trapped. It can also be used to alter the environment around the target, therefore confusing them. And of course, a really important use of it is to extract information from people. But it does not cause them any physical pain, it is merely used to manipulate or brainwash the target.
There are four types of Genjutsu that I know of. There is Normal Genjutsu, which can be performed by anyone and is the one that I will be learning from you. There is also Sound Genjutsu, which requires the user to have sound as their ability. This is probably the deadliest of the Genjutsu's since it does not require eye contact to be made. So, it could even be performed on a blind person! Also, Sound Genjutsu can be performed from a further distance. Then there is Sharingan Genjutsu, which of course requires the possession of Sharingan, which I do not have. And lastly, there is smell based Genjutsu.
Genjutsu is most effective when it is used at a close range from the target. The further away, the weaker the Genjutsu will be. This is why it is rare for it to exceed mid range. Genjutsu also requires a lot of skill to perform, as the user must be intelligent and also have a very fine level of chakra control. And to produce a successful illusion, the chakra must be molded in a certain way, which is why the hand seals are required.
So when a person is trapped in a Genjutsu, it does not necessarily mean that everything is over for them. You see, there are ways with which Genjustu can be escaped from. The first step to break or counter a Genjutsu would be to actually identify that one is trapped in the illusion. Otherwise, it will be meaningless. After that, there are a few ways with which one may escape. One way is to disturb or stop the flow of chakra in the body, which will cause a spike in the chakra. This will cause a disturbance, releasing the technique. Another way would be to cause pain to your body. Someone else may also be there to disrupt the flow in your chakra, which can also get you out. And a Bijuu can also get their Jinchuriki free from the illusion, if they are on good terms with them, that is. And the last way to counter would be for a person with a dojutsu.