From the knowledge that I can muster up now I would say that genjutsu can be simply defined as mind tricks. The opponent releases their chakra and sends it into your body, this then lets them manipulate your five senses however they please. By an example of its applicability I hope you mean a time when it has been used in the anime, if so. One time I can think of is when Itachi placed Kakashi under his mangekyo sharingan's genjutsu, tsukuyomi, this made Kakashi think that he was stuck in a world where seconds on the outside was days in that world. It also kept him still in the genjutsu and on the outside. Finally unlike regular genjutsu it actually manipulated Kakshi's pain receptors and made him feel pain every time Itachi stabbed him in the dream world.
Hope that's good enough because that's really all I know.