X-heads vs J-heads: what's the difference??

X-heads vs J-heads: what exactly is the difference between the two?

I tried searching the tech forums for the answer but I'm obviously not very good at getting the most out of a keyword search (that's another story).

Anyway, I'm not sure what the difference is between these two heads. I use to know the difference by casting numbers but nobody seems to refer to them by their casting numbers anymore like they do with the big block heads. Can someone school me on the "X" and "J" heads and the differences between them, or point to me a thread that may have already beat that horse to death?