Hitchcock is also known for his use of the McGuffin. What a McGuffin you ask? From Wikipedia:

In fiction, a MacGuffin (sometimes McGuffin or maguffin) is a plot device in the form of some goal, desired object, or other motivator that the protagonist (and sometimes the antagonist) is willing to do and sacrifice almost anything to pursue, often with little or no narrative explanation as to why it is considered so desirable.

The MacGuffin is common in films, especially thrillers. Usually the MacGuffin is the central focus of the film in the first act, and then declines in importance as the struggles and motivations of characters play out. It may come back into play at the climax of the story, but sometimes the MacGuffin is actually forgotten by the end of the story.

I'm back again. An example would be that everyone is running around looking for microfilm - but they never tell you what's on the microfilm. In a recent DeNiro film "Ronin" everyone is trying to steal a briefcase but the contents of the briefcase are never revealed.