The metaphor I find best to describe Postmodernism is that of the Tower of Babel.

Begin with humanity back in the dawn of language, where people are simple trying to come up with a means of describeing reality as it is. Reality is what happens to be around them and so, in order to make reality a little easier to live with, they begin to make a means to deal with it, to suit it to their purposes. They build a simple, one story structure on top of reality, just to make life easier. This way, they can deal with the structure rather than with the harsher reality.

However, the structure itself is not enough. People now have a basic way to communicate reality to themselves and to one another, but they must now not only describe the reality that was outside, but also the reality that INCLUDES the structure. The system is still course. So, in order to more accurately define reality, in order to live more comfortably, in order to understand reality more fully, they must ADD a section between themselves and the ground. Between themselves and the world at base.

This continues as with each new story, the full nature of reality and human consciousness/interaction is modified slightly. At first there's no trouble. Each new story in the tower manages to roughly approximate and refine those below. But eventually, the stair from top to bottom gets very long, the connecting passages between rooms and stories are complicated by the need to make each new story fit structurally on top of the old stories. And one day, people find that they can no longer find their way from their houses at the top of the tower to the true base reality at the bottom. The world has become too complicated. The only thing keeping them trying is the knowledge that somewhere, something must be holding the tower up. This stage is called "modernism," and was ushered in more or less by the First World War and the realization that the old way of doing things might not be the best way.

Then comes the trouble. As people build ever higher, they begin to realize that most of the structure rests on top of other structure, and that... maybe there isn't any base at all. The long tower that had an abstract but unknowable base at one time, now no longer has and support at all, and the entire thing collapses into a single, massive, half constructed warren, in which EVERY point is the center. Or rather, NO point is the center.

Post Modernism is the result of the delicate balance of Modernism, the surety of people that there MUST be a meaning somewhere beyond the gibberish of discourse, collapses and nothing is left but the gibberish defining itself. Nothing has meaning, and communication about and solid reality becomes truly, impossible.