In order to see what post-modernism is, we have to know what modernism is. "Modern" simply means now, current, up-to-date, etc. So what could POST-modernism be if we are always living in the modern age (as opposed to the Middle Ages, or the roman empire, etc)?
The best I can do to give you an idea about what I think post-modernism is, is to give you some examples of things that are post-modern.
Before photography, which is a "modern" invention, artists would paint realistic paintings of people and places. The idea was to make it as exactly like reality. A "mirror" of reality. (Notice that this is similar to one idea about how the mind works. Some think the mind is a "mirror" of the real world.) But then with photography, the artists had to do other things, not paint reality, but something else. Art started out as a reflection of reality and has now ended up as a hyper-reality that is totally different than the "real world", (just like how some say there is no objective "real world")
Also with modern technology you could copy paintings. Some believed that the price of an original paining would drop in value because one could just get a copy. But that didn't happen. The POST-MODERN result has been that the original is valued even more than the copy. Before recorded music, the only way to hear music was live. But just because you can buy a CD of your favorite band doesn’t mean you don't want to see them in person does it? In fact, seeing them live is MORE valuable than before. This is the POST-MODERN result.
My favorite example is natural breasts. Women have always had natural breasts, so why the big deal now? Because of the MODERN invention of silicone breasts. Now the "real thing" is more valued. But it's not a new thing really, it's an old thing, but it takes on a different look and is so "new" again.
Anyway, I am against intellectual elitism that has infected our present day post-modernism. There is no good reason to exclude large parts of the population to enlightening ideas simply to feel superior. If anything the last few years have shown us, it's that with the organic spread of the internet, we've seen a growing pluralist intellectual society emerging, and to a greater extent an explosion of entrepreneurial pluralist elite (the Dells, Gates', 20yr old silicon valley millionaires) whom curious aren't exclusivist, but rather quite inclusivist.
In defining post-modernism, even people who ascribe to some of the values that are said to be post-modern do not consider themselves as such and avoid trying to define what post modernism actually is. Albeit, the opposition strangely seems to have a definitive definition of what it is.
The various tenets I ascribe to is the multiplicity of perspectives, and contrary to popular modernist impositions of what they think I believe, I don't think that most postmodernists believe in the absence of a kind of objective reality, only that, like Heisenberg's Uncertainty Principle, we change the things we perceive simply by observing them, more so through the processing of the images and the consequent judgement of them. The hamburger is not a cow. In fact, it is probably several different bits of cows.
What was once a modernist whirlwind of ideas, with people like T.S. Eliot (A true elitist), Ezra Pound, Keats, Joyce, etc., standing by the sidelines, waiting to extract from it the various kernels of truths, the moment of access, or revelation, now has scattered. And the contemporary equivalent to the modernist vortex metaphor would be a rain of intellectual truism, on which anyone can connect any number of droplets in any arrangement, imposing whatever image of truism they want.
So, what's your perspective?