Genjutsu are techniques that use the chakra in the victim's brain to form illusions, it falls into the category of Yin release. They normally require you to weave hand seals, however there are a few exceptions, for example Uchiha usually only use eye contact while operating Genjutsu. Genjutsu will use hypnotism to cause hallucinations in others, thus allowing the user to hurt and play with the victim's emotions. Kurenai and 2nd Mizukage are few genjutsu users who are not part of the Uchiha Clan. Sensors are able to know whether they are within a genjutsu or not due to their visual prowess, intelligent characters who have studied the way Genjutsu works also wouldn't have much problem discovering whether they are in a Genjutsu or not. To combat the utilisation of Genjutsu you can stop the flow of chakra in your body and then apply a superior power to disrupt the flow of the original user's chakra, this is Genjutsu Kai. Another option is to have an outside ninja unaffected by the Genjutsu make body contact, and use their chakra to disrupt the opponent's chakra flow, but some jutsus which include of the sharingan jutsus such as Tsukuyomi and Kotoamatsukami may still be difficult to break, due to them being instantaneous.