to love the man they desire to love. Both novels end tragically for the historical "star-crossed lovers."

One final example of intertextuality is any literary text which adopted for film. Any filmatic adaptation which has been influenced by an original literary text is an example of intertextuality. The original text is read, interpreted, defined and adapted according to the allocated meaning by the writer.

Many examples of this exist: Kenneth Branagh's adaptation of Mary Shelley's Frankenstein, Neil Jordan's adaptation of Anne Rice's Interview With a Vampire, and Elia Kazan's adaptation of Tennessee Williams' play A Streetcar Named Desire.

Essentially, the question is asking how the knowledge of one text, which influences another, has helped one to understand the text in question. As defined above, one can see how texts shape other texts. This is how intertexuality helps one in reading. The effect is generally more subtle as themes, style, tone, minimalism, genre, etc., or literary period also influence texts that come later or in contemporaneous periods.

The previous post was dead on accurate.  I would only like to respond with my own examples of intertextuality.  Certainly, the idea that no work is created in a vacuum from other works that influence it is something that is at the basis of intertextuality.  Ironically, I think that philosophy is a discipline that lives and perishes with intertextuality.  Philosophers are constantly referring back to other works that influence it either in support, or in opposition, or valences in between.  The Greek thinkers end up being the starting point for most philosophic texts and then it grows from there.  For example, when reading a modern philosopher like Richard Rorty, one actually ends up pulling from Greek sources, Enlightenment thinkers, and other post modern thinkers.  This is what makes intertextuality so powerful.  In literature, the works of thinkers like Gabriel Garcia Marquez and Carlos Fuentes are profoundly impacted by the works that come out of Spain's Golden Age, such as Cervantes' "Don Quixote."  Milan Kundera reflects this in a more intensified form, often pulling from Rabelais, Nabokov, or other writers in asserting his own point of view.  Sometimes, his references are so direct that one has to stop and ensure they are clear on the intertextual point being made.  The idea that "we all stand on the shoulders of giants" is brought out by intertextuality because it proves the interlinking nature of all thought.  As a closing example, I recently realized this in answering a question on John Rawls here on enotes.  When one reads John Rawls "A Theory of Justice," one is not reading only Rawls, but many political philosophers that preceded him.

Intertextuality is complex and involves different levels.  I'll try to simplify it a bit. 

Basically, it is the idea that no text is independent, that all texts are amalgams or mixtures of texts that have come before them.  A text may be directly influenced by another text, or may indirectly use language and ideas that have been previously established by other texts. 

For instance, the Anglo-Saxon epic Beowulf is related to what came before it--Greek epics.  Shakespeare used numerous sources to write Macbeth and Hamlet that are well documented.  Milton imitates Greek epics in Paradise Lost.  The romantics drastically reacted to the neoclassicists and rejected virtually everything the writers who immediately preceeded them believed--and rejection is a reaction:  it's intertextuality.  The moderns reacted to the Victorians, the postmoderns to the moderns, and on and on.  No text is isolated. 

Concerning the second aspect of intertextuality, any literary work that deals with good and evil, for instance, is dealing with ideas that have been previously established.  There's a reason why The Lord of the Rings trilogy seems medieval and somewhat Old Testament-like.  It's been said that every love story written since Shakespeare owes a debt to his Romeo and Juliet.  And love stories, of course, did not originate with Shakespeare. 

At its simplest, intertextuality is evident everytime a novel is made into a movie.  The novel and the movie are both texts, of a sort.  Allusions are also simple examples of intertextuality.  As pointed out in my favorite literature handbook, The Bedford Glossary of Critical and Literary Terms, when a character on Lost is shown reading an actual novel, that is an allusion.  It is intertextuality.