S-rank

Shinobi
An S-Class shinobi is a ninja who has gained power and knowledge far beyond an average Jonin and have earned a legendary, or feared status. The title itself isn't an official rank and the village officially refers to them as Jonin, and is more of a casually-applied term instead of a formal title. Some ninja who have achieved this rank sometimes leave their villages to seek their own ambitions, such as most of the members of the Akatsuki, thus becoming missing-nin and having no rank at all. All of the major ninja villages put their S-Class missing-nin in the Bingo Book, marking them for assassination for all Jonin and ANBU ninja. Most ninja however adopt a "flee on sight" policy rather than to fight one. Most Kage's tend to be S-Class, and sometimes a team of S-Class ninja would be given nicknames by their allies and enemies, such as the Sannin.

Missions
S-Class missions are the missions with the highest difficulty. They pose great risk because the ninjas can meet great number of skilled opponents (Jonins or S-Class shinobis) and from this reason, the only ones who can carry S-Class missions are highly experienced Jonins or a team of ninjas.