"Breaking the fourth wall" is a phrase that originates from theater, wherein the audience represents the invisible fourth wall of the room wherein the play is set. Thus, any time the characters within a story address the audience either directly or indirectly, the fourth wall is broken.

How about Lion King? You know the song "Hakuna Matata"? When Timon says, "Shush, Puumba, not in front of the kids," that is breaking the fourth wall. The kids are those watching the movie. In comic books, whenever Deadpool references that he is a comic book character or talks about Batman (who is in another comic book universe and doesn't exist in Deadpool comics) or even talks directly to the reader, that is breaking the fourth wall. When Ferris Bueller faces the camera and talks to the viewer, that is breaking the fourth wall.

Here's the TV tropes article on it:

http://tvtropes.org/pmwiki/pmwiki.php/Main/BreakingTheFourthWall

Careful, though; TV tropes is addictive.
-Quaddy

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