My understanding of postmodernism is that its a culture.
This question is best answered using a context.
In art: Post modernism is a blurring of what was accepted as art in the past and what is art now.
Jasper Johns exhibited his bed - an original Jasper Johns, the world went 'oooh, aaaah' and then he made another bed, exactly the same as his own bed and exibited that - thus proving that art can be replicated.
Warhol took this to an extreme by making copies of everyday items and displaying them in art galleries.
Are things displayed in art galleries art because they ARE are or because they are in an art gallery? Warhol (and Johns) thought that they became art because they were accepted as art when exhibited.
In modern culture we have an excellent example of postmodernism in the cult of celebrity. Celebrities used to be people like movie stars, the royal family, models etc... Now people are famous for no other reason than being famous. Jade Goody is an excellent example of this, she is photographed and written about - but she did nothing of any talent to become famous, she is literally famous for her fame - which began in a reality tv show. Reality tv shows are also an example of postmodern culture. TV shows used to be made based on reality but acted by actors, now we have reality shows, people who just want to be famous. And normal people sit at home in their own reality watching someone elses reality on tv!!
In short, postmodernism is the blurring of the lines of reality, the concept that nothing is original anymore, everything that can be done has been done so now art, music, tv etc.. are moving onto things that have been done - but doing them in a different way with everyday objects, everyday sounds, everyday people.
Now Im even more confused...