If you didnt know much about the ranks of ninja feel free to read the lore here.
Academy students (忍者学校生, Akademī-sei) make up the student body of the Academy, where they are trained and prepared for life as a shinobi. They are not an actual part of the shinobi force, as they are still in the process of mastering the very basics of the ninja lifestyle. However, they can be conscripted as part of the war potential in times of emergency.
Academy students spend their time honing the various skills they are required to have and undergoing various tests and exams to see how they progress. Besides ordinary school subjects such as history and mathematics, they are taught the basics of ninjutsu, taijutsu, and most likely also genjutsu. This includes learning about chakra and how to control it, tactics, hand seals, they are taken through the handling of ninja weapons and tools, like shuriken and kunai, learning how to throw and wield them through target practice, and several E-rank techniques, such as the Clone Technique and the Transformation Technique. They are also trained in working both individually and in teams.
Eventually they are given a graduation test to see if they are ready to try and become genin. Passing this test does not automatically grant them this title, as they need to pass a second test given by their jōnin instructor, but they do receive a forehead protector. Those who fail either of these tests are sent back to the Academy.
Genin (下忍; Literally meaning "low ninja", Meaning (Viz) "junior ninja") are the lowest level of ninja and also the ones that display the most difference in power. When they become genin, ninja start to do their bit for their village's economy – being sent on missions that the village gets paid for. They are typically sent either on D-rank missions, which are almost entirely risk-free jobs of manual labour, or, rarely, on C-rank missions, which are a cut above that and begin to verge on real "ninja" work that have a very low possibility of risk to the ninja involved. When Konohagakure was short-handed on upper-level ninja because of the invasion, the village had to occasionally send genin on higher-ranked missions.
Chūnin (中忍; Literally meaning "Middle Ninja", Meaning (Viz) "Journeyman Ninja") are ninja who are qualified to guide other ninja and lead missions. Chūnin have reached a level of maturity and ability that primarily consists of leadership skills and tactical prowess. Genin who do not have the skills to become chūnin are weeded out in the chūnin exams. Some of them, like Iruka Umino, Daikoku Funeno and Suzume move on to function as Academy teachers and others, like Shikamaru Nara, serve as team leaders in charge of small teams who need to make decisions and utilise the skills of the shinobi under their command to the maximum effect. Chūnin are typically sent on C-rank or B-rank missions.
The ANBU (暗部; English TV "ANBU Black Ops"; Literally meaning "Dark Side"), short for Ansatsu Senjutsu Tokushu Butai (暗殺戦術特殊部隊; Literally meaning "Special Assassination and Tactical Squad"), take orders directly from the Kage, performing special high-level missions, such as assassinations and torture. The ANBU usually work in teams formed to the requirements of the mission, ensuring maximum success. The shinobi in the ANBU are hand-picked by the Kage; chosen for their individual capabilities and special skills. Age, gender, background, or previous rank bear no weight in this decision. Members can also leave the organisation, as Kakashi did for unknown reasons to return to the regular forces. They wear their mask even in their own village, for their identity as an ANBU agent is to be known only by the Kage and village elders
Jōnin (上忍, Jōnin; Literally meaning "High Ninja", Meaning (Viz) "Elite Ninja") 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. Jōnin are generally able to use at least two types of elemental chakra, some genjutsu, and above average taijutsu skills
A Kage (影; Literally meaning "Shadow") is the leader of one of the five most powerful hidden villages and are generally acknowledged as the most powerful ninja in their respective villages. They are collectively known as the Five Kage (五影, Gokage; Literally meaning "Five Shadows").