Itachi, Pain, Konan, Sasori, and Deidara were all S-Class. They could each singlehandedly devastate an entire village all on their own. They are also able to subdue Bijuu in pairs (or singlehandedly also).

So I'd assume they are quite a ways stronger than even your more talented Jonin. 
I figure it would be like comparing Kakashi as A-Rank and Itachi as the S-Rank. 

Wikia really doesn't offer much information on A-Rank though, unfortunately. But, looking at Madara, Obito, Nagato and Kakazu, it seems like S-Rank would be a ninja that exceeds the strength of an individual Kage and, as an individual, is a threat to an entire village all on their own. 

That's my take on the difference. :0