Auteur is the French word for Author.

Auteur theory is a theory of filmmaking in which the director is viewed as the major creative force in a motion picture, and it arose in France in the late 1940s.

Auteur theory states that the director is the “author” of the movie more so than is the writer of the screenplay

The director is the author because s/he oversees all audio and visual elements of the motion picture. In other words, such fundamental visual elements as camera placement, blocking, lighting, and scene length, rather than plot line, convey the message of the film.

Auteur theory argues that the most cinematically successful films contain the unmistakable personal stamp of the director.  Basically, the director provides a unique personal artistic perspective, and the resulting film is a product of that perspective.  

The employment of the French word for "author" (auteur) associates the director with the individual output of other mediums (painting, literature, etc.), and rebels against the seeming "collectiveness" of the film studio.  

Francois Truffaut's sensational assertion that "there are no good and bad movies, only good and bad directors" importantly situates film as the characterized output of a creative individual, and responds to the economic and productive tensions in filmmaking.

 This places the director as the creator in the personal sense that is found in other forms of art.

Another very conscious function of the theory is the emphasis on a director’s body of work as a whole, rather than individual films.  This is a product of the notion that the "imprint" of the director will be apparent, regardless of a particular film's consistency or relation to other films in the individual's collected body of work.  In short, placing a film within a chronological context becomes important in the auteurist sense.

Thus, even if the subject matter of any auteur’s works may be vastly different, the choices for camera work and for mise-en-scene can be easily identified as belonging to a single creative persona.