Over the years, investigations into things like such as Watergate, 911, the JFK Assassination, Clinton/Lewinsky etc., have all been headed by a supposedly independent investigator. The titles of those investigations and others have ranged from Commissions, Independent Counsels, Special Counsels, and Special Prosecutors; all to do essentially the same thing.
It is interesting to me, that Democrats are now calling for a "Special Prosecutor" regarding the investigation of Russian meddling in our presidential election and possible collusion by Trump and/or his surrogates rather than calling for an Independent Counsel or a Special Counsel. So why the words "Special Prosecutor"? Well, it implies that a conclusion of wrong-doing has already been reached. Here's the definition of a prosecutor: "a person, especially a public official, who institutes legal proceedings against someone." Of course, the firing of Comey only helps solidify Trump's guilt in the minds of the Democrats. Thus they feel, more than ever, a Special Prosecutor is needed.
It is all about the Democrats making the voting public believe Trump is already guilty by simply choosing the words that imply it. Once again, its all about politics, since after months of FBI investigations, no Trump-collusion with the Russians has been found, stated or leaked.
Senate Dems: Hey, Maybe We Won't Confirm a New FBI Director Until a Special Prosecutor is Appointed: https://townhall.com/tipsheet/guybenson/2017/05/15/senate-dems-hey-maybe-we-wont-confirm-a-new-fbi-director-until-a-special-prosecutor-is-appointed-n2326728
Definition: Prosecutor: https://www.google.com/search?q=prosecutor&ie=utf-8&oe=utf-8
CNN: The public really wants a special prosecutor for Russia investigation: http://www.cnn.com/2017/05/15/politics/donald-trump-russia/