This is a great question. There are a number of ways in which to define post-modernism. First, post-modernism is what comes after modernism, which is a belief in the objectivity of knowledge and progression through technology and science.
Post-modernism, then, is the belief that knowledge is no longer objective. Along with this, post-modernism does not believe that there is one meta-narrative that joins everything together. Michael Foucault, for example, talks about the archaeology of knowledge. By this, he means that all people and countries have their own stories and these stories are just a valid as someone else's story. Why? There is no objectivity. knowledge is relative.
Post-moderism also is very keen when it comes to power. It studies how power is used and abused.