ECC and registered are 2 totally different things. ECC is error checking and correcting, registered RAM the opposite of unbuffered. I really don't know what exactly registered is though, or how it works.
ECC RAM is about 5% slower than non-ECC RAM - however it is recommended if the applications you are running are very important (if you are doing something regarding nuclear bombs on your computer, you don't even want the slightest chance of an error in a memory module)
But both ECC and registered are most common in server boards, like the guy already said