Postmodernism is the term used more often in art and literature.
Poststructuralism is the term more often used in philosophy and in the theory OF art and lit.
Poststructuralist theory is interconnected with politics and sort of has to be understood in context. If you try to assess and evaluate it in any kind of "pure" sense (i.e., "I am not so interested in who espouses this stuff or what they're up to when they do so, that's all ad hominem distractionary stuff, just let me see what the theory itself says") you end up holding nothing in your hands and going "wha....???"
It is anti-theory theory. It says that it would be impossible to come to any body of theory in any kind of neutral attitude anyhow (you aren't capable of it) and aside from that there CANNOT BE any meaning embedded in any body of theory that can be separated out from the political intentions of those who crafted it, nor any reading of it that is not overdetermined by the attitudes and intentions of the reader.
Everything is a freaking
discourse, a sort of semantic tug-of-war in which adversarially poised factions gain or lose power, and do so via the attempt to define things, shape the meaning of things, etc.
And there are no "subjects" (i.e., you are not here; you may think you are but who "you" are is just another such battlefield; you are always already overdetermned by your location in space time and cultural context, totally a product of your environment).
Since nothing can be known (everything you think you know is a product of your own overdetermined preformed perspective in conjunction with the loaded political slantings of every presentation by every participant in the discourse) there is no inherent meaning to BE known. Nothing is intrinsically "better" in any shape way fashion or form, whether the venue be artistic, political, moral, gastronomic, or even raw nerve endings ("does this feel good or bad when I jab you with this poker? well that's an outcome of your location in social spacetime... ").
So at this point you should be seeing what I mean when I say it all slides through your hands and that you'll end up saying "But... but there's no THERE there in this 'theory', it's empty!"
So, on to those who espouse it. Which (in theory) is a line of inquiry that they'd approve of since their own theory SAYS their own theory HAS to be empty (since everything is), so with their blessing...
They are for the most part post-Marxist academic lefties, and the name of the game is to pry open the established "canon", i.e., whatever body of collected wisdom or repository of long-recognized Great Works, to make room for artists, musicians, poets, authors, and etc who come from the "margins", the left-out folks. It's all one massively verbose argument that amounts to "NO inherent 'best of class' for ANY category of ANYTHING exists; it is ALWAYS ONLY what You, and People Like You, happen to like; and that has more to do with who you are than what the thing is". With me so far? Can you see the utilitarian value for someone trying to get Toni Morrison or Aboriginal Dance or the culinary accomplishments of the Inuit entered in alongside of Shakespeare, classical ballet, and French/Continental cuisine?
Now, you may be thinking "But that's a rather stupid way of going about it. It lets you argue that the works of Toni Morrison is as much a great body of world-class literature as Shakespeare's plays, but at the expense of not being able to argue that Toni Morrison's writing is any better than whatever the worst student in Freshman English 101 has written ABOUT her work as a term paper". And you would, of course, be dead-on correct.
They don't care. This theory is not accidentally opaque and hideously godawful to parse. It is
deliberately that way. Remember, they do not believe that their own theory has any content. They USE their theory TO WIN AN ARGUMENT and it bothers them not in the least that to be true to their own theory they would be unable to grade one student's crummy essay any different than any other, or to choose to teach Toni Morrison instead of having the whole class study last semester's freshman term papers as works of great lit. Their students can't plow through the theory well enough to figure OUT that this is what it says, and the purpose to which the theory was put to use was to attack Allan Bloom and folks of his ilk who wanted students to be receiving a classical education and wanted excellence to be acknowledged and pointed to, and mediocre efforts (as they saw it) by less excellent (but, ahem, pretty good for One of their People you have to admit) authors and artists, etc. And to win control over what gets taught to the freshmen.
There was room to argue that excellence exists, but that perhaps we can't
easily detach ourselves from our cultural background to see excellence produced from other backgrounds; there was room to ensconce Toni Morrison as a great author (without an invisible tip of the hat to blackness or femaleness being better represented by shoehorning her in). They don't care. In by the good door, in by the bad door, who cares as long as you get in? Those moldy academic arguments are just posturing and bullshit and everything is power struggle anyhow.
I could go on at great length (as if I hadn't already) about what else these folks don't care about (theory itself, for example) but you can probably take it from here on your own.