Genjutsu are illusionary techniques requiring handseals. 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 , however, there are plenty of other uses depending on the situation. They are not used as often as ninjutsu because they don't physically harm the opponent. Genjutsu can also be used to manipulate others, similar to brainwashing by feeding the victim illusive suggestions.
I have not yet had any training in this..