Your rank as a ninja determines everything from your range of strength as well as which gaming features you have access to!
As well as having requirements, each rank has stat caps which can be viewed here.

• 18,000 Experience / Academy Student Level 10
• Level 5 Clone, Replacement, and Transform Technique
• 12 intelligence

1. Sensei: a Jounin, Special Jounin or Commander can now add you as his/her student.
2. Bank: you can now send ryo to other players as well as save your own ryo.
3. Avatar: you can now put a picture on your profile page for others to see.
4. D-rank missions: you can now do D-rank missions.
5. More jutsu options.

• 90,000 experience
• 250 total defense
• 500 total offense + defense
• 20 intelligence

1. Combat: you can now attack other Chuunin and Jounin. They can attack you, as well!
2. Change Villages: you can now leave your village, and/or join a different village.
3. ANBU: you can now join or even lead an ANBU squad.
4. C-rank missions/crimes: you can now do C-rank missions/crimes.
5. More jutsu options.
6. Marriage feature which allows you to marry another in the game, so long as they are also chuunin+.
7. Medical Ninja and Bounty Hunter occupations.
8. Serious crimes.

• 40,000 total defense
• 60,000 total offense + defense
• 2,000 total general stats

1. Combat: you can now attack/be attacked by Chuunin, Jounin, and Special jounin.
2. Kage: you can now challenge the Kage of your village for his/her position.
3. B-rank missions/crimes: you can now do B-rank missions/crimes.
4. More jutsu options.
5. Sensei occupation.

• 120,000 total defense
• 240,000 total offense + defense
• 4,000 total general stats

1. Combat: you can no longer attack/be attacked by Chuunin. However, you are now able to attack/be attacked by Commanders.
2. A-rank missions/crimes: you can now do A-rank missions/crimes.
3. More jutsu options.

• 200,000,000 exp
• 1,600,000 total defense
• 2,000,000 total offense + defense
• 450,000 total general stats

1. Combat: you can no longer attack/be attacked by Jounin.
2. More jutsu options.
3. S-rank missions: you can now do S-rank missions.
4. The ability to create your own clan.

In TheNinja-RPG, it is common to rank hold. i.e. stay at a lower rank even though it is possible to rank up. This is done for many reasons, but is most often done because of features and stat caps unlocked and forfeited due to ranking up.

• sebihp2007's Rank Holding Guide (Old)