Well, here's what the SACE Stage 1 English Subject Outline has to say about it:

Intertextual study

Students reflect on their understanding of intertextuality by:

• analysing the relationships between texts, or

• demonstrating how their knowledge of other texts has influenced the creation of their own texts.

When analysing or creating texts to show their understanding of intertextuality, students may also consider:

• intertextual references within texts (texts that make explicit or implied references to other texts)

• ways in which they, as readers, make intertextual connections based on their previous experiences of texts or their own experiences and beliefs.

Potential areas of study could include:

• two or more texts by the same author

• two or more examples of the text type (e.g. two film trailers or three editorials)

• a text type and a mode of communication covering the same topic

• different text types about a similar idea, designed to meet the needs of a particular audience

• a text that is based on or makes reference to other texts

• hybrid texts, such as infotainment or product placement in film.

Well, again, according to SACE, it's this:

The shaping of a text's meaning by the reading of other texts or the interrelationship of texts such as when an author borrows from or transforms another text or a reader’s referencing of one text in reading another. Texts gain meaning through their reference to or evocation of other texts.

In other words, it comes down to:

• The deliberate connections that might exist between two texts

• References an author makes to other texts within their own text

• Similarities you as a reader/viewer find between the text being studied and other texts you might be familiar with.

Confused?

Don't be. We'll look at some examples and you'll probably find you've been doing this whole 'intertextuality' thing for years without even realising it.

And here's a clip explaining it:

There are different types of Intertextuality you should be aware of:

• Intertextuality: relationship between/across texts

• Appropriate (with the stress on the last syllable): use another’s work within a text

• Subvert: undermine another’s work within a text

• Allusion/allude: indirect referencing to another text (eg the title of the film Stranger Than Fiction alludes to a quote: “Truth is stranger than fiction, but it is because Fiction is obliged to stick to possibilities; Truth isn't.”― Mark Twain, Following the Equator: A Journey Around the World)

• Parody: imitation for comic effect (you can't make fun of a text without making references to it)

• Mashup: combining elements of other texts to create a new one. Here are a couple of examples:

Firstly, The Simpsons have been using Intertextuality for decades, so there are some really good examples to use. Consider the following two clips:

• Firstly, here's a link to an animated version of Edgar Allan Poe's short story The Tell-Tale Heart (don't worry about the German writing on the opening credits - the audio is in English).

• Secondly a clip from The Simpsons season 6, episode 2 'Lisa's Rival' that makes direct references to Poe's story to help tell its own story of the pressures of a guilty conscience.

And here's a Simpsons couch gag making use of a certain well-known fantasy TV show's famous opening credits sequence purely for comic effect:

And one more example from The Simpsons. In season 26 they invited awesome Mexican director Guillermo del Toro to direct the opening credits for their Treehouse of Horror episode. He packed it with dozens of references to  movies ranging from classic horror, sci-fi, fantasy, a number of his own films and even a few past Simpsons episodes. See how many you can spot:

Did you spot them all?

Here's a version with all (or, at least, most) of them identified and labelled.