Jonin (上忍, Jōnin, Literally meaning: High Ninja, Meaning (Viz): Elite Ninja) are generally highly experienced and highly skilled shinobi who serve as military captains. Jōnin are generally able to use at least two types of elemental chakra. It is not unusual for jōnin to go on missions alone. They are often sent on A-rank missions, and if experienced enough, S-rank missions as well (which are considered to be the greatest difficulty).
To become a jōnin, one may be recommended. If enough villagers and the respective Kage deem the shinobi to be fitting to the rank, they will be promoted. In the anime, another way to become a jōnin is to participate in the Jōnin Exam.
When a ninja becomes a jōnin, they may be assigned to supervise a three-man genin team. Their duty will be to instruct and train them to become competent ninja. They also can decide when the genin are ready to participate in the Chūnin Exams.
In Konohagakure, it is up to the jōnin to decide whether a candidate Hokage would be given the position permanently. They also have a special rank known as Jōnin Commander. Shinobi who have jōnin-level skills in only a specialised area are classified as Tokubetsu Jōnin.