1) Genjutsu is that of you 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 thing his senses are capable of pain, heat, cold, tickles, iching, visions, sounds, etc. A ninja can make the opponent taste, smell, feel, see or hear anything he wants. However, because this is done by using your chakra to manipulate his chakra flow, the ninja needs to be close or, many times, have a direct link towards the opponent such as eye contact or whatnot.
2) Some Genjutsu users would be Yakumo Kurama, Tayuya, and Kurenai Yuhi. Their common trait would be that all three of them excel at Genjutsu and is mainly their primary ability that they use in battles.
3) Well Genjutsu alters your opponents 5 senses so it would be a good way to make your opponent do certain things or help you avoid certain actions made by your opponent. That is pretty much the reason why I want to learn Genjutsu and pretty much how I would use it in battle.