Here's how AFI describes their poll:
# Movie Quote - A statement, phrase or brief exchange of dialogue spoken in an American film. [AFI defined an American film as an English language motion picture with significant creative and/or financial production elements from the United States. Additionally, only Movie Quotes from feature-length American films released before January 1, 2004 were considered. AFI defined a feature-length film as a motion picture of narrative format that was typically over 60 minutes in length.]
# Cultural Impact - Movie Quotes that viewers used in their own lives and situations; circulating through popular culture, and becoming part of the national lexicon.
# Legacy - Movie Quotes that viewers used to evoke the memory of a treasured film, thus ensuring and enlivening its historical legacy.