What is a "show runner"? (Simpsons DVD commentators)

Several of the people on the commentary tracks of the Simpsons on DVD are show runners. What do show runners do?

You know, I just heard that term when I was watching the commentary on Arrested Development- but they actually explained it. :wink: Sounds like the show runner is basically in charge of the whole writing process, making sure different scripts are coming along, that kind of thing.