What is postmodernism and how do postmodern thoughts impact various fields such as arts, religion, politics, business, and the media?

I'll try to answer another part.  In terms of the media, the pervasiveness and the global nature of 24 hour news is part of the fragmentation which is another characteristic of postmodernism.  Fragmented, meaning decentralized: this is partly a description of the fragmented self where the postmodernist does not think of him/herself as one Main thing, but many things, sometimes even paradoxical.  To tie this into the media, where news is always on and all over the world, a sense of being stuck in one part of the world is not so stuck any more.  With so much, albeit indirect, access to other cultures, we're more integrated and therefore, less walled up in our own respective nations and selves. 

Todd Gitlin once described postmodernism as the need to be everything all at once.  As the previous poster noted, postmodernism rages against rational and scientific aspects.  Jean Lyotard, in The Postmodern Condition, talked about postmodernists "raging against metanarratives." A metanarrative is exactly what it sounds like a Big Story: examples being Christianity or any religion, Modernism, Technology, and any group of coherent ideas that supposes itself to be an ideal plan for society. 

So, summing this all up, for the postmodernist, there is no One Really Good Idea, no One Best Religion, no One Best Artform.  The postmodernist takes bits and pieces from all these "grand narratives" and meshes them all together: sometimes in art this is called "pastiche."  So, not only is the self fragmented but the stories, theories, religions are also fragmented, decentralized.  To be everthing everywhere all at once is, without strict adherence to One Main or One Oppressive idea is to be a postmodernist. And tying in to one of the previous poster's bullets, a self-conscious or self-reflexive postmodernist would make it known that he or she is, for example, a Buddhist Marxist Cubist Feminist Gun-loving Republican Vegetarian with ethnic roots in Liberia, England and Mauritania.  Everywhere and fragmented: a paradoxically futile and hopeful attempt to be a citizen of the world.