I think its a Chicago thing... It's beef cooked with italian seasonings (garlic, parsley, oregano, basil, marjoram, thyme), sliced thin, and then heated back up in an au jus made from beef stock seasoned with the same type of seasonings. Its then served on an italian or french roll with either hot peppers or sweet peppers cooked in the beef stock mixture. Sometimes you'll see it covered in mozzerella. Around here you can order it four ways; Dry (no juice, just the beef and the roll), Regular (beef and a little of the juide), dipped (they dip the roll in the juice for a couple of seconds) and drowned (where they make the sammie and then hold it under in the juice unil the bubbles stop.
Just like anything else, there is a debate in Chicago as to who has thde best beef sammies. The Portillo's chain do a great job and are very consistent. There is a new chain called Buona Beef that is very good, but Al's Beef Shop is still the winner in most surveys.
Too much info?