I believe basically it's certified mail with a return reciept request, meaning it's delivered to them and someone has to sign for it - then you get that info back either physically or via internet tracking depending on what you specify.
Some use the tracking, but the party line around here is to go CMRRR, so that you get that green card back with a name and signature on it. You have to go to the P.O. in order to see who signed for it, if you track it. I personally like CMRRR.