Showrunners in Television Writing
A showrunner's job is pretty much to oversee every aspect of the show's development: sitting with the staff and generating story ideas, trying to get network and studio approval on those ideas so that they can "go to script" (be assigned to a writer, or team of writers to flesh out in script form), personally ensuring the quality of every script that goes to the network and studio brass (even if it means several late-night rewrites for the showrunner, which it usually does). Then it is the showrunner's job to make sure that the cast is happy with the script and entertain the (all too frequent) complaints from cast members regarding a particular episode's treatment of their character.
A showrunner for a one-hour show will then occasionally go off to the set (from a local studio soundstage to a remote location in Canada) to check on the actual physical production; a sitcom showrunner, however, always oversees physical production as it tends to take place on a soundstage right below the writing offices (sitcom writers, as a whole, tend to work longer hours than their one-hour writing counterparts, primarily because the writing tends to be done more collaboratively, but are not, for the most part, paid a higher salary). Showrunners want to ensure that the director's work is consistent with the show's other episodes so that the show does not appear to have (depending on the number of directors on staff) five or so entirely different looks. Showrunners work late nights and frequently weekends as well -- it's safe to say that a showrunner's job is never done, at least until hiatus.