jōnin are generally highly-experienced shinobi with great individual skill who serve as military captains. They are often sent on A-rank missions, and experienced jōnin may even be sent on S-rank missions (which are considered to be the greatest difficulty). It is not unusual for jōnin to go on missions alone.
It is as of yet unknown what one has to undergo to become one. It is mentioned that jōnin are appointed, When a ninja becomes a jōnin, they may be assigned a three-man genin team to supervise. One part of their duties is to help instruct and train these genin to become competent ninja. In Konohagakure, it is up to the jōnin to decide whether a candidate Hokage would be given the position permanently.