From Hidden Frontier
The title of Executive Producer (EP) typically describes a film producer or television producer who doesn't participate in the technical operations of the production process, but who is still responsible for the success of a project.
In the production of a television show there may be many Executive Producers listed. Sometimes it may be a situation similar to that found in motion picture production - someone with previous involvement with a particular work, a project's financier, or someone in control of the business aspect of production. Sometimes, this title is conferred upon a celebrity or notable creator who has lent their name to a project to boost its prestige or credibility, as a recognition of newly-acquired industry status, or as a perk to the show's main star or creative force. The principal writer for a show is almost always credited as an Executive Producer rather than the more descriptive title of "Head Writer" under the unusual rules for establishing writing credits on television series (where writers are often credited as "Producers").
In television, an Executive Producer might have more power than any other credited crew member — this is the case with the "show runner". Generally, a show runner (still credited as an Executive Producer) is the creator of a series, or an influential staff writer on the show. His/her role is to guide the overall creative progress of the show's story over the course of a season — basically, the de facto creative director of the show. All creative decisions (from casting to script approval) go through them. Usually, while there may be many credited "Producers" and "Executive Producers" (with varying responsibilities) on a television series, there are just one or two in the position of show runner.
For these reasons, it is not unusual for shows to have three sets of Executive Producers: "Traditional" EP's (Production Executives, Financiers, etc.), Head Writer(s), and show runner(s).