During the late 70's and early 80's, George Lucas, a filmmaker who scrutinized every detail of his films and embraced technology as means of improving the quality of films, became dissatisfied with existing movie sound quality.
In an effort to standardize and improve the quality of movie soundtracks, special effects and dialog, Lucas set out to define a set of guidelines that would enhance the typical movie viewing experience.
Named after his first feature film, THX 1138, THX systems were installed for the premiere of Return of the Jedi.
Not to be confused with Dolby Digital or DTS or Dolby Pro Logic. THX, as stated above, is merely a certification and has nothing to do with the actual decoding of sound.