The McRib is to pork what the McNugget is to chicken - no big surprise. Its a "boneless" rib. How the hell is that even possible? Boneless pigs? Maybe. See http://www.bonelesspigs.org/.
No, this is one of those things that you'd never eat once you know what is in it and how its made.
This little piggy is made from pig innards, like tripe, heart, and scalded stomach. Its ground up and mixed with the dregs blasted off the ping carcass with a high pressure hose. The resulting slurry is then compressed, dried and formed into the rib shape. Its then deep fried before being coated in a disgusting sauce to hide what it really is.
Then its put between the bun from hell, made using various chemicals and a flour-bleaching agent used most commonly in the manufacturing of foamed plastics like gym mats and the soles of shoes.
So eat up! The McRib is only back for a limited time!