I'd like a succinct comparison of the two "-isms", though I know this is a tall order.
There are a few things that postmodern literature does differently than modernist literature:
- Doesn't seek or offer solutions to problems. Modernist writers tackle the chaos of life and generally emerge with at least a partial promise of hope. Postmodern writers rarely imply that the world will ever become less absurd than it is. Ex: Vonnegut's Cat's Cradle presents a completely broken society with no possibility for salvation.
- Has no specific meaning. This doesn't mean that it has no meaning at all, but rather the reader has to create the meaning through interpretation. Postmodern literature rarely has any didactic qualities like a "moral to the story." When there is a given "meaning" in a work, it is often accompanied with a contradicting or inconsistent counterpart. Ex: Calvino's Invisible Cities portrays lots of different interpretations of what a city is, all of them equally valid but inconsistent, and most have no single explicit "point."
- Symbols and tradition are incorporated in order to tear down their meaning rather than to build from it. Borges' Pierre Menard, Author of the Quixote famously uses the text of Don Quixote in order to change its meaning entirely.
- Lacks an ending in any traditional sense. The famous final line of Eliot's modernist poem Love Song of J. Alfred Prufrock "Till human voices wake us, and we drown." could never exist in postmodern writings. Postmodern writing rarely has any conclusion, putting the burden on the reader to put the pieces together. Ex: Pynchon's Crying of Lot 49 which presents an overload of information throughout the novel but never fully explains its significance, or Vonnegut's Slaughterhouse-Five which simply ends without any climax or satisfying conclusion.
- Plays with the structure of the text. Ex: Danielewski's House of Leaves incorporates vital plot information in the footnotes, appendixes, and index of the novel. Also think of genres such as altered books, graphic novels, or interactive fiction which are all notable for their creativity of their structure.
This is by no means a complete list, but it covers many main points.
Some critics, such as Annie Dillard in her book Living By Fiction, argue that post-modernism (in literature) is actually just "contemporary modernism", and that there are no significant differences between them.
Note that, for example, most of the points raised in the answer by @jbpjackson above would apply to Joyce's "Ulysses", which is sometimes categorized as the pinnacle of modernism.
Also note that I am speaking strictly about "modernism" and "post-modernism" in their literary sense; the application of these terms in different domains (such as architecture or philosophy) is quite different.
Modernism is characterized by a loss of faith in the "transcendental signified"—or a kind of generalized loss of faith—yet unlike postmodernism is still somewhat nostalgic for the time when that faith was intact. Postmodernism is further decentered.
While modernism blurs the distinction between high and low art, postmodernism rejects it.
However, in an addendum to his influential book, The Postmodern Condition, Jean-François Lyotard argues that every work of art is, in fact, post-modern at the very moment of its production:
A work can become modern only if it is first postmodern. Postmodern, then, is not modern in its end, but in the nascent state, and this state is constant.
Below are my edits of 2 answers from Reddit for readability. Posts 1-3 are less formal explanations.
Post 4. Overview.
Modernism: "Our society is outdated! Religion is unnecessary. We need new ideas, let's think out of the box!"
Post-modernism: "There is no truth, there is no reason, there is no morality. Everyone is oppressed and it is society's fault! I'm offended!"
Post-postmodernism: Sorry, have no clue.
Post 5. Formal explanation
Modernism and Post-Modernism are extremely broad terms that describe mass cultural movements in Western art and philosophy. To understand them, one must also understand the general evolution of Western philosophy, especially the concepts of Rationalism and Romanticism.
Modernism is an extension of Romanticism (which was a revolt against Rationalism).
Post-Modernism is a critique of Modernism.
I don't know much about Post-Post-Modernism (what a name), as I believe it’s still infantile. Now the long explanation!
I'll try to start with Rationalism. Rationalism developed through the Renaissance and the Enlightenment, and encouraged the use of empirical observation and reason to determine the truth of the universe. A quintessential example of Rationalist thought is Descartes and his famous conclusion, "I think, therefore I am". In brief, Descartes theorized that human beings may not necessarily be able to trust their own senses — you may see, hear, or touch an object, but how can you trust that what you see is the truth, rather than a very elaborate illusion? Thus he uses deductive reasoning to determine the first thing that can objectively be determined as real — because you think, you therefore MUST exist as a real, and not illusory, entity. Descartes is thus using deductive reasoning to determine what is objectively real.
Romanticism was a reaction to the changing society of 19th-century Europe — industrial and scientific developments were quickly rendering ancient institutions and social orders obsolete. Romanticism was thus a reaction against the values of Rationalism that had been crucial in developing the modern scientific method and propelling that progress.
Romanticism was also far more of an art movement than a true philosophical movement. It evoked imagery of a romantic past, and emphasized the use of meta-narratives to justify an argument. It focuses more on the intangible than the tangible. For example, a common theme of the Romantic movement was that scientific progress disrupts the natural order and is thus dangerous and volatile — Mary Shelley's Frankenstein concerns this theme, and is one of the most famous works of the Romantic movement. It is from Romanticism that many intangible modern political concepts spring (e.g. Manifest Destiny, Ethnic Nationalism, Zeitgeist, "Human Spirit").
Between the movements overhead and beneath, Realism developed as an art movement, a reaction to Romanticism. It emphasized realistic depictions of its subjects and sympathized more with the lower strata of society and new developments, rather than the upper strata and an idealized past.
Modernism was an evolution of Romanticism. Modernism utilized meta-narratives, but also encouraged the criticism of everything, including the old order. It rejected Realism. Where Romanticism was more concerned with the dangers of human progress and a return to an idealized past, Modernism was more concerned with finding out where wayward progress went wrong and finding out what is "holding back" true progress. As Modernism developed as an art movement, it encouraged the use of more abstract forms of creative expression — hence the stereotype of "modern art" being incomprehensible.
World War I hugely influenced the rising Modernist movement. WW1’s horror and destruction showed how dangerous and destructive modern industrial warfare could be. But WW1 also represented a huge shake-up of the European political order — old monarchial empires were toppled in favor of democratic republics or communist states. And so as a political movement, Modernism examined the meta-narratives surrounding Western society — what holds back social progress, and how can that change.
It is from this that we see the rise of social movements against perceived oppression: progressivism, socialism, feminism, civil rights, popular sovereignty. These are all movements centered around a meta-narrative of one group being oppressed in some way by another - laborers by capitalists, women by men, minorities by a majority, ethnic groups by other ethnic groups.
Post-Modernism developed as a result of the trauma of WW2, and is primarily a critique of Modernism. A central part of Post-Modernism is the inherent unreliability of human judgment — humans are not inherently rational creatures, and will commit irrational self-defeating actions if they believe those actions will benefit. Post-Modernism thus emphasizes the inherent unreliability of those Modernist meta-narratives that are formulated by humans, and are thus an unreliable source. An effective example of how this relates to WW2, and the inherent unreliability of meta-narratives, is the Nazis themselves — the Nazi ideology heavily relied upon the use of extreme (and false) nationalist and racist meta-narratives to justify itself.
Thus in both Post-Modernist philosophy and art, you’ll see a heavy emphasis on criticism of institutions and their foundations. Overcoming oppression on the surface-level is not enough — we must examine and change the factors and flaws deep within our culture and society to progress as a species. Thus you get emphasis on concepts such as moral relativism in Post-Modernism than in Modernism.
I'm more familiar with the artistic side of Post-Modernism, and can give several examples of Post-Modern art. One of the most famous is "The Treachery of Images". It is a painting of a pipe, with the caption reading "This is not a pipe". The image is not lying — it's not a pipe, it's the painting of a pipe. But when you look at it and are asked what it is, you wouldn't say "That's a painting of pipe". You’d likely say "That's a pipe". I hope this isn't too abstract. Essentially, the image encourages the observer to question their assumptions about what they are seeing.
Post-Modern film and television tend to examine and critique the tropes and norms of their genre or medium (as a Post-Modern social movement would critique the norms and values of their culture). For example, a big part of Scream's appeal and popularity was its willingness to extensively explore and make fun of the conventions of the slasher genre, which had been run into the ground by the mid-1990s. Buffy the Vampire Slayer examines the conventions and clichés of the horror and high school drama genres, and is positively soaked in existentialist philosophy.
And in literature, a good example of a Post-Modern book series is A Song of Ice and Fire, which brutally deconstructs the conventions of the high fantasy genre. And Deconstruction is a key weapon in the arsenal of the Post-Modern artist or activist - examine a convention or norm, explore its unfiltered effects and origins, and try to make it different.
As for Post-Post-Modernism, again I am unfamiliar, but because it’s a reaction to Post-Modernism I assume it has to do with rejecting the relativist and deconstructionist beliefs of the Post-Modernists.
Post 6. Criticism of Post #5.
Post #5 is a good broad-strokes summary, but minor points I'd add: 1) A big part of postmodern is arguably the collapse of 'low culture' and 'high culture'; you'll often see deliberately jarring mish-mashes of trashy pop and old, vaunted works, for example. This is in part a reaction against the modernist instinct towards making everything very Serious and Difficult and Meaningful (postmodernism, in a word, is playful.)
2) Your explanation of the term 'deconstruction' is true in the sense that postmodern works often do this, but I’d argue that it differs from what Jacques Derrida was saying when he promoted the term. In fairness, it's difficult to figure out what he’s saying, but my take on it is this:
Every text (a piece of fiction, or architecture, or dance, or hand-gesture, or whatever) is fraught with tensions and self-contradictions. When you examine it closely enough, any text will self-implode into meaninglessness, deconstructing itself. As such, anyone telling you that the meaning of Moby-Dick is so-and-so is entirely wrong, because Moby Dick doesn't have 'a' meaning. It either has no meaning at all, or it has a hundred meanings, a thousand, infinite shards of fractured meaning, all jutting against each other and not fitting into one cohesive whole. I hope this makes some sense. Peter Barry's book Beginner Theory is the best way for a clear and accurate understanding of Derridean deconstruction.
The answers are directed post modern and modern literature and not to theory. The modern age is generally thought of as starting during the Renaissance with the rise of the age of reason in world views and analysis. The post modern age has been said by some to have started with the end of world war 2 - when 2 non-European armies met for the first time in Berlin after conquering all of Europe, putting an end to the many wars that European countries had waged against each other for many centuries. If I remember correctly most deconstruction theory, which is considered post modern, arose after this time.