No, sorry, same thing; it’s sometimes spelled one way and sometimes the other. I’m trying to get used to spelling it IS-2, but I’m used to spelling it JS-2.
It stands for “Joseph Stalin 2” (in standard English romanization) which is also spelled Iosef Stalin in a more literal/accurate romanization, I believe. Either way, “ole Joe Stalin” is the same person and the second model of tank named after him is the same tank.