Show Runner

In short, the executive producer who serves as the head writer. The longer answer is that the showrunner is the person who gives the show its tone and direction.

The initial showrunner is usually the creator (frequently the writer of the Pilot). As the years pass, the original showrunner may leave and a new one may join - this can often be a Jump the Shark or Growing the Beard moment.

Creator/Showrunners:

 * David X. Cohen: Futurama, with co-creator Matt Groening.
 * Tina Fey : Thirty Rock. Also plays a showrunner on the show.
 * Bryan Fuller: Pushing Daisies
 * Howard Leeds: Small Wonder
 * Gene Roddenberry:
 * The Lieutenant (1963-64)
 * Star Trek: The Original Series (Seasons 1-2 only)
 * Star Trek: The Animated Series (de facto)
 * Star Trek: The Next Generation (Season 1 only)
 * Paul Simms: News Radio
 * David Simon: The Wire and The Corner (He also wrote the book on which Homicide: Life On the Street was based, and later joined the writing staff.)
 * Aaron Sorkin: Sports Night, The West Wing, Studio 60 On the Sunset Strip
 * Joss Whedon of Mutant Enemy
 * J. Michael Straczynski: Babylon 5
 * Matt Groening, James L. Brooks & Sam Simon: The Simpsons (season 1 and 2)
 * Matthew Weiner: Mad Men
 * Mitchell Hurwitz: Arrested Development
 * Shawn Ryan: The Shield
 * Ryan Murphy: Brad Falchuk and Ian Brennan (collectively known as RIB): Glee
 * Eric Kripke: Supernatural
 * David Crane and Marta Kauffman: Friends
 * Larry David and Jerry Seinfeld: Seinfeld (David left after season 7)
 * Kurt Sutter: Sons of Anarchy
 * Buck Henry: Get Smart. (Co-creator Mel Brooks had little involvement with the series after the pilot)
 * Danny Arnold: Barney Miller
 * Dan Harmon: Community
 * Dan Schneider(y'know Dennis from Head of the Class): ICarly, Drake and Josh, and Victorious
 * Amy Sherman-Palladino: Gilmore Girls(seasons 1-6)
 * David E. Kelly: Ally McBeal, The Practice, Boston Legal
 * Marc Cherry: Desperate Housewives
 * Shonda Rhimes: Grey's Anatomy and Private Practice.
 * Norman Lear: All in The Family, Maude, Sanford and Son.
 * Stephen J. Cannell:The Rockford Files, The A-Team, The Greatest American Hero
 * David Chase: The Sopranos.
 * Steven Bochco: Hill Street Blues, LA Law, NYPD Blue
 * Bryan Konietzko and Michael Dante Di Martino: Avatar: The Last Airbender and its Sequel Series The Legend of Korra.
 * Jeri Taylor: Star Trek: Voyager (jointly with co-creator Michael Piller for seasons 1-2; left after season 4)
 * Rick Berman and Brannon Braga: Star Trek: Enterprise (Seasons 1-3)

Showrunnners Who Took Over Existing Shows

 * Russell T. Davies: Doctor Who - Series 1-4 + 2009 Specials.
 * Steven Moffat: Doctor Who - Series 5 onwards.
 * Al Jean & Mike Reiss: The Simpsons - Seasons 3 & 4
 * David Mirkin: Seasons 5 & 6
 * Bill Oakley & Josh Weinstein: Seasons 7 & 8.
 * Mike Scully: Seasons 9 through 12.
 * Al Jean: Season 13 to present.
 * Tim Minear, who took on the role for Joss Whedon on some shows, as well as being primary creator/showrunner on some other short-lived outings.
 * Journeyman producer Fred Freiberger became showrunner for the third season (often referred to as the turd season) of Star Trek: The Original Series, and became the scapegoat for all its problems.
 * Ira Steven Behr took over Star Trek: Deep Space Nine from Season 3 onwards, leading to much facial hair growth.
 * Maurice Hurley became showrunner in the second seasons of Star Trek: The Next Generation and Baywatch Nights. His work on the former helped the show move towards fully Growing the Beard, but his efforts on the latter proved that no matter how badly a show starts out, it can still Jump the Shark spectacularly.
 * Michael Piller became the showrunner for the third season of Star Trek: The Next Generation, and held the role for the rest of its run. He also acted as showrunner for the first two seasons of Star Trek: Deep Space Nine, and then joint showrunner (with Jeri Taylor) of the first two seasons of Star Trek: Voyager.
 * Brannon Braga became the showrunner of Star Trek: Voyager for seasons 5-6, departing in season 7 to develop Star Trek: Enterprise. Like Freiberger before him, he became a major scapegoat, and is often blamed for all of the problems with the show's creative direction throughout its entire run.
 * Kenneth Biller became the final showrunner for Star Trek: Voyager in season 7.
 * Manny Coto took over primary showrunning duties from Rick Berman and Brannon Braga in the fourth season of Star Trek: Enterprise, replacing Braga as executive producer. Although season 4 was generally considered the best one, it was too late to save the show from cancellation.
 * Writer/executive producer Sera Gamble is taking over as showrunner for Supernatural as of Season 6.
 * Neal Baer became showrunner of Law and Order Special Victims Unit for season 2, replacing original showrunner Robert Palm, and has remained in that position for eleven seasons, the longest tenure in franchise history. He will be moving on at the end of season 12.
 * Rene Balcer assumed the role of showrunner for the original Law and Order twice: once in the late '90s, and again in the late noughties. In both cases, he left to helm a spinoff as the initial showrunner: First Law and Order: Criminal Intent, and then Law & Order: Los Angeles.

Shows Where the First Showrunner Was Not the Creator

 * Lost is a hybrid as far as the creator/showrunner classification is concerned. Created/showrun by J.J. Abrams and Damon Lindelof and run since early on by Lindelof and Carlton Cuse.
 * Doctor Who, which has only had a 'showrunner' position since the 2005 revival. Created primarily by Sydney Newman, Verity Lambert, Donald Wilson and C. E. Webber. The original series may be said to have divided the showrunner's duties between the producer and the script editor; sometimes the former (eg. John Nathan-Turner from 1980 to the cancellation) or the latter (eg. Douglas Adams, 1979; Andrew Cartmel, 1987-89) seemed to have more of the role.
 * Likewise, Torchwood and The Sarah Jane Adventures were both technically created by Russell T. Davies, but the initial showrunners were Chris Chibnall and Phil Ford respectively. Davies later took full control of Torchwood starting with its third series, while Ford remained showrunner on The Sarah Jane Adventures for its entire run.
 * An odd example in Wizards of Waverly Place: The show's creator, Todd J. Greenwald, remains on-staff as a writer and producer, but he is not the showrunner - that honour went to Peter Murrieta for the first three seasons. (In the fourth season, he left and Greenwald stayed on - but Greenwald still did not become the showrunner.)
 * Edward Allen Bernero wasn't the creator of Criminal Minds, but was its showrunner from the pilot to the end of the sixth season.
 * If a film series can count, producer David Heyman is essentially the showrunner of the Harry Potter movie franchise. Directors come and go and Heyman is often the one who selects them.

Showrunners in Fiction

 * Thirty Rock: Liz Lemon
 * Studio 60 On the Sunset Strip: Matt Albie and Danny Tripp
 * Almost Perfect: Kim Cooper
 * The TV Set: David Duchovny plays beleaguered showrunner Mike Klein.

Long-Running Shows That Never Changed Showrunners

 * Curb Your Enthusiasm: Seven seasons, showrunner Larry David. The main character? Larry David, a showrunner.