''Well first off..'' Kazu said before he spoke about genjutsu.
''Genjutsu is a illusionary art that can manipulate the opponents chakra which causes said individual to undergo varies effects such as hallucinations, pain, loss of hearing and vision, or even death. There are two types of genjutsu, the passive and the non chakra fueled types. Types like effects that disable vision and hearing or even altering the factor, and the other kind that uses Doujutsu. Genjutsu works by using chakra to disturb the chakra flow of the opponent, specially in the brain, altering his senses. By doing this you can make him perceive any you wish through senses, sounds, vision, pain, temperature and other things. The user needs to be close as well because the difficulty of effect is determined by the range, so if the user is close it is easy to induce a genjutsu and the longer or far away they are from the target the harder it is to induce a genjutsu.
Everyone has a balanced chakra flow and when a genjutsu is attempted or successfully used the balance between the two tips and twists and curves to a unbalanced area. To break this effect is to inflict pain on yourself and to perform this with accuracy is to do as listed:
D Rank Genjutsu: Chipping a Finger (like one does as he bites to draw blood for a Summoning Jutsu)
C Rank Genjutsu: Low level Physical Attacks, Bitting a chunk of your Cheek
B Rank Genjutsu: Low ranked Taijutsu/Freestyle
A Rank Genjutsu: High ranked Taijutsu, Stabbing Yourself in minor muscle, Cutting yourself, Breaking Smaller bones, Low level Burning/Voltage, Painful Cold
S Rank Genjutsu: EIG/Sage level Taijutsu, Breaking Bones, Deep Stabs or cuts, Medium Voltage/Burning, Extreme Painful Cold, Bitting your tongue off
Some Genjutsu can create a complete sensory manipulation to a point that while the illusion is undergoing in the targets mind, in reality, he's "frozen". This happens because the user creates an illusion that completely replaces the targets senses, leaving him to react in his mind to the illusion. Some, however, only block or mess with one sense, eyes, ears, mind, extra which leaves the target receiving the rest of the real sensory information. As such, not having been pulled into the illusionary world of the Genjutsu, the user can in this case move, react and act in reality.
There are multiple ways of detecting a genjutsu, however one way is to be smart enough to know your under one like the act of seeing way to many copy of someone which is not a actual jutsu or is even legal. Other ways is to detect the change of flow within the chakra which is harder, however it is a reasonable way to know if your under a genjutsu. Everything with genjutsu is determined by the rank and skill of which it is used, weak genjutsu is basically not even a genjutsu at all or even a waste of your chakra for the fact that it is so easy to reverse or detect. However, genjutsu can be used to easily win a fight as well which is also determined by skill. Everything genjutsu is is skill to make it short.
Altering the balance between physical and spiritual energies in your chakra by disturbing your physical energy. This is done by producing pain. Pain comes from the body and thus it makes your physical energy fluctuate by "shocking" your body. This will spike your chakra into resetting thus returning the balance of chakra.
Altering your chakra flow through the introduction of foreign chakra or energies. This can be done by an ally who is capable of using the chakra transfer technique or by entering, for example, sage mode or being a jinchuriki in control of your bijuu or releasing a Curse Mark. The reasoning is that as foreign chakra enters your system or another energy, your chakra flow becomes altered and unbalanced and thus releases the genjutsu. This enables the release of any Genjutsu except Mangekyo Sharingan grade Genjutsu.
and genjutsu layering casting is just like ''What'' to me... ''