A high level Ninja who is usually in charge of training a group of Genin, and are often sent on A and S-Rank missions, sometimes even going alone.
Jonin are highly-trained elite ninja who take on various roles throughout their respected villages. Some of these Jonin who are elite and well-trained are capable of training a group of three genin. Kurenai,Asuma, Kakashi, and Guy are all Jonin who take these specific roles. There are also special ninjas that are Jonin only because they excel in a specific field, such as Genjutsu. These types of Jonin are known as Tokubetsu Jonin.
It isn't exactly known how exactly a ninja becomes a Jonin, it has been mentioned that one can be appointed, and there is an exam. Also, Jonin have a special job of deciding if a canidate for Hokage get the position or not.