first would be that both movements reject the sense of totality.  Modernists believed that the presence of human freedom and autonomy were forces that could reject the basic tendencies of Enlightenment and Romanticism visions of harmony.  The scientific and rational foundations of the Enlightenment were rejected because modernists discounted the objective truth was possible or attainable.  They also rejected the Romantic visions of harmony and unity in that the modernists were convinced that fragmentation and disunity were the only true elements in the world and within individual consciousness.  The modernists sought to create and compose art in this setting, reflecting these ideas.  The expression of these ideas took form through literature, and while modernity was severely criticized, the form of doing so was still within the traditional boundaries of what had been done prior.  In this light, modernists sought to criticize the setting in new and innovative ways through traditional form.  Postmodernism sought to move modernism to a new level by criticizing everything, including the form.  The postmodernists believed in the ideas of fragmentation and disunity, but sought to explore the principles that lay underneath such a reality.  This led to a complete overhaul of both content and manner in which it is presented.  Beckett's "Waiting for Godot" is a play that radicalizes what the play genre can include.  Ionesco does the same.  Thinkers like Foucault sought to underscore and analyze the basic premises of power and its construction in society.  Postmodernists sought to transcend language and traditional forms of expression of self.  Yet, in order to articulate this, some level of standard form was needed, so while postmodernists really sought to do something different, there is much within it that had to represent a starting point, so that there is not a complete repudiation of what is there, but rather a smaller version of resurrecting the death in order to kill it.

I have done a little research into this topic so that I could give a more detailed answer (OK, I’m a modernist, not postmodernist!). What I have discovered is that there is a lot of disagreement over the meaning of these terms but the bottom line is that postmodernism is against almost all the tenets of modernism.

For example, modernism believes in rational thought while postmodernists believe everything is irrational. Modernism places a strong emphasis on science, whereas postmodernists are anti-scientific. Modernists believe that there are universal values and tend to be somewhat optimistic whereas postmodernists believe that only local values have importance. Modernists favor organization; postmodernists believe life is chaotic and fragmented. Modernists reflect the values of European or Western thought whereas postmodernists believe in multiculturalism. Modernists are objective; postmodernists are subjective. Modernists believe that life is purposeful; postmodernists believe that life is meaningless or that meaning is purely subjective and relative. Modernists believe one can define morality whereas postmodernists believe morality is relative.  Modernists are analytical whereas postmodernists are rhetorical. Modern art is characterized by simplicity, elegance and streamlined design but postmodern art is decorative and elaborate. Modernist philosophy is determined by cause and effect but postmodernists believe in chance. Modernists are linear thinkers; postmodernists are circular thinkers or haphazard thinkers. Modernists believe in permanence; postmodernists believe in transience. Modernists believe that truth is objective; postmodernists believe that truth is relative and socially constructed. Modernists are not very political whereas postmodernists politicize everything.

Postmodernism, as the name suggests, is a reaction against modernism, defining itself as something opposing and superseding the previous movement. Modernism was the response of various artists to modernity, and it was most often a negative and antidemocratic response. John Carey, in The Intellectuals and the Masses, summed up the attitude of highbrow modernist writers to their readers when he said that only one of them, James Joyce, wrote about the common man, and he wrote in a way that the common man could not read. Joyce, T. S. Eliot, Virginia Woolf, and many other modernists were obsessed with making their texts challenging, obscure, and allusive.

Postmodernists reacted against the modernist aesthetic by challenging this intellectual aristocracy and its claims to truth and beauty. While modernism emphasized exclusiveness to the point of minimalism, postmodernism aimed to be democratic and inclusive, mixing popular art indiscriminately with high art. Rather than focusing exclusively on the artist as a solitary figure, postmodernists saw the writer and the reader as collaborators in creating meaning. Writers such as John Barth, Joseph Heller, Thomas Pynchon, and Kurt Vonnegut wrote sprawling, chaotic narratives drawing on an eclectic range of influences.

Although the postmodern novel aimed to be uniquely contemporary, these authors also looked back long before modernism to such precursors as Rabelais, Cervantes, and Sterne. A similar dichotomy between modernism and postmodernism can be observed in art forms other than literature, particularly painting and architecture.