What is that ring that i see every slytherin wearing?

Its the Slythrin House ring! Its a status symbol. Just like a Family crest ring অথবা a class ring from অক্সফোর্ড is a status symbol. And we all know how much Slythrins are preoccupied with status...(especially blood status-which is synomnous with the house of Slythrin)

I think it's the slytherin crest. I've seen some of the old boarding schools that have houses do something like that.

That's the Slytherin crest ring. To প্রদর্শনী your Slytherin pride !