S-Rank is one of the six classifications. It is preceded by A-Rank. It is typically used when classifying techniques intended for Kage, arunin, and highly experienced jonin level shinobi. S-Rank techniques are almost always unique to a single user, and as such are trademark abilities of that user.
There are also S-Rank missions, the highest paying and most dangerous type of missions a ninja can go to. These missions are exclusively assigned to arunin, highly skill jonin, or large squads of ninja. S-Rank missions pay at least 1,000,000 ryo.
"S-Rank" is also a designation given to extremely powerful rogue ninja, earning them a place in their village of origin's Bingo Book. Each member of the Akatsuki is an S-Rank criminal. Threr are some classify powerful ninja who are still loyal to their village as S-Rank, such as the Sannin or other Kage-level characters, but this is arbitrary.