The ninja in a hidden village have their own skill levels which help portray what rank they are. The most talented ninja in the village is generally the Kage who leads the village, the lowest tier is made up of those young individuals training in the ninja academy. Although there are variations from village to village, the general organisational structure and hierarchy of the ninja systems of each village is about the same. Listed below is the ranking from lowest skill level to highest.
Groups such as the ANBU and Sand Assassins are not a level, but rather a specialized unit made up of ninja.
Academy Students are not really considered part of the ninja force. They make up the student body of the Academy, where they are taught about the basic techniques and about the ninja world. Once they graduate from the Academy they will be promoted to the rank of Genin and will join the regular ninja forces.
Genin is the lowest ninja rank attainable, they tend to make up the general mass of their villages. Although considered the lowest rank, their are still those that rise up and challenge even those of higher ranks. They can be sponsored by their Chunin and up sensei for the Chunin Exam.
Specialized Jonin tend to not follow the general path of the hierarchy. This rank is rewarded to those who show great skill in one particular area but are not quite ready to follow up into higher tiers. They will either be treated as a Team Leader or a Soldier depending on who is available at the moment.
Chunin were formerly Genin who were promoted for displaying excellent intelligence, strategy and combat skills in the Chunin Exam. Chunin have the equivalent rank of military captain(outranking Specialized Jonin), using their knowledge and wisdom, they will lead teams in times of war. These ninja can also become field doctors or teachers.
Jonin are generally highly skilled and experienced ninja. They know the ins and outs of the ninja world and therefore are credible to be Commanders during war times. Currently being one of the highest ranks attainable, they are shown great respect among their peers.
Kage the strongest ninja in the hidden village. The Kage serves as the pillar upon which the village stands and the strength a village represents to the other ninja villages largely depends upon the strength of the Kage in power. Both Leaf and Sand have a Kage.