Utopias afford consolation: although they have no real locality there is nevertheless a fantastic, untroubled region in which they are able to unfold: they open up cities with vast avenues, superbly planted gardens, countries where life is easy, even though the road to them is chimerical. Heterotopias are disturbing, probably because they shatter or tangle common names, because they destroy "syntax" in advance, and not only that less apparent syntax which causes words and things (next to and also opposite one another) to "hold together." This is why utopias permit fables and discourses: they run with the very grain of language and are part of the fundamental fabula: heterotopias desiccate speech, stop words in their tracks, contest the very possibility of grammar at its source: they dissolve our myths and sterilize the lyricism of our sentences.
Michelle Foucault, The Order of Things

A heterotopia, a concept introduced and expounded on by Michel Foucault in his books The Order of Things (Les Mots et Les Choses, literally Words and Things), The Archeology of Knowledge (L'achéologie du Savior), and, my favorite discussion, in the essay This Is Not a Pipe (Ceci N'est Pas une Pipe), is a place (real or fictional) that subverts representation. Representation is the connection between the symbol and the symbolized. A symbol can represent its subject by one of multiple possible ways, but let's focus for now on symbols that represent their subject by resembling them. Think of a movie actor, Leonardo DiCaprio say, portraying a real person, Howard Hughes; or think more simply, of a picture of a pipe, representing, presumably, some real pipe that exists somewhere or existed some time. Or for that matter, think of a movie actor, DiCaprio in The Departed this time, portraying a non-real person. The latter is still within the usual scope of symbols, and is typical of what Foucault calls the fundamental fabula.

Well, a heterotopia is full of apparent symbols that are very similar to other things (Foucault distinguishes between similitude and resemblence), that beg of you to think of them as representing something, but as you follow the arrow leading from the pseudo-symbol to what you have expected it to symbolize, you are lead to a dead end, or on a wild-goose chase, or around and around in circles; you are subverted. Whereas in a utopia, the subject of the symbols might not exist in reality, in a heterotopia the subject often does not exist at all. A heterotopia mocks you for expecting the existence of an apparent copy (simulacrum is the word Foucault's translator uses) to imply an original (real or fantastic). Thus, heterotopias subvert the use of symbols, that is they subvert language, "desicate speech, stop words in their tracks, contest the very possibility of grammar at its source".

What I am describing is the idea of a heterotopia as it applies to art and literature, but Foucault and others also use it to describe various institutions and social constructs. The latter use is not one I understand very well, and I will leave it to somebody else who wants to write it up.

Foucault's example of a heterotopia in This Is Not a Pipe is René Magritte's paintings. However, my favorite example of the idea of a heteroptopia, as compared to a utopia, is Tom Stoppard's play Rosencrantz & Guildenstern Are Dead, as compared to Samuel Beckett's Waiting for Godot. In Waiting for Godot, we have two acts, two days in the life of Didi and Gogo. Each day someone named Godot, for whom they are waiting, is supposed to show up but doesn't. It is implied that these two days are two in an endless series stretching endlessly back and forward. All of these days, simulacra of each other, are devoid of substance, composed of idleness and conversation-as-filler, as Didi and Gogo are paralyzed in their inability to ascertain any a-priori knowledge, or indeed to figure out what they're meant to do. The reason I call this a utopia is that it is grounded in the notion of a law of the conservation of meaning inherent in the rules of representation: because, to Beckett, the endless copies of Didi and Gogo's lack an original, Didi and Gogo cannot have any originality. If they find their original, namely if Godot shows up, their life will be affirmed, but because there's no expectation he will, their life is meaningless. You might like to call it a dystopia instead of a utopia (hasn't Utopia always been a dystopia), but in any case it is very indicative of the premise of existentialism.

On the other hand, in Rosencrantz & Guildenstern Are Dead, we have another couple of characters who are trying unsuccessfully to get a grip on their existence. Ros. and Guil. run around behind and in front of the scenes of Hamlet, trying to figure out what the hell it is that they're supposed to be doing here, going on very little information (they're not even sure which one of them is Ros. and which is Guil.). Miraculously, they stumble into all of their scenes and utter all their lines. The way Ros. and Guil. try to figure out what they're meant to do is not to look for their supposed original (the script), but to figure out what people expect them to do.  One of the question the play raises is "are Ros. and Guil. following the script or is the script a result of Ros. and Guil.'s actions?" Indeed Stoppard puts into his play even more levels of similitude -- apparent representation -- (and even gets some layers for free with his choice of Hamlet, which already has in it a play within a play,) and all of these copies in turn provoke more questions about the direction of the arrows of representation between simulacra. But these are all rhetorical questions without any satisfactory answers. They simply point out the absurdity of the whole endeavor, the attempt to derive affirmation from representation.

Rather the contrary seems to be implied: the sheer number of different ways in which we might choose to understand the ambiguous ways the simulacra interact contests understanding. This is a heterotopia --  "the disorder in which a large number of possible orders glitter separately" (The Order of Things). Whereas Waiting for Godot is a broken utopia, a chain of arrows pointing one to the next, never reaching a destination, Rosencrantz & Guildenstern are dead is a playful answer: a whole mess of arrows pointing any and everywhere, spinning randomly. Rosencrantz & Guildenstern Are Dead is great when it makes fun of the futility of discourse in this heterotopia that it inhabits: Ros. and Guil.'s vain atempts to ascertain a-priori facts like their own names, the direction of the wind, or why the heck a tossed coin keeps coming up heads invariably become hilariously absurd. Stoppard makes a mockery of Beckett's fatalism. In his heterotopia, attempting to derive a personal meaning through philosophical discourse doesn't make sense. Instead, the advice one acting troupe member gives to Ros. and Guil., indeed that Stoppard gives to any of us, is "Relax. Respond. That's what people do." This is postmodernism's response to existentialism. This is the ultimate application of  the concept of heterotopia, to one's philosophical conception of the world.