POSTMODERNISM

1. Since the Sixties on.
2. It questions the idea of authority, centre and absolute truth.
3. It doesn't believe in the existence of an absolute meaning because meaning is never stable and it is always slippery, still to come, differed.
4. It abolishes the difference between low and high art.
5. It uses different types of language according to the different situations humans have to face.
6. Postmodernist literary works are products of intertextuality.
7. It believes in the concept of relativity.

MODERNISM

1. It covers the first 3 decades of the century.
2. It still believes in the possibility to find truth, it looks for an abstract truth of life.
3. It looks for a final destination.
4. It constructs a coherent world-view.
5. Modernism is based on using rationality.
6. Literary works were considered as unique creations of the artists.

Postmodernism is a cultural movement which developed after the Second World War, especially since the Sixties on. "Post" means after and it also may mean a development of Modernism, which covers the first three decades of the century. The suffix "ism" implies something negative because everything new is considered negative by people (people is against changes).
The most important concept of Postmodernism is that it questions the idea of authority, truth and centre, differently from Modernism which still seems to believe in the possibility of finding a truth something to believe in.
Another important concept of Postmodernism is that it doesn't believe in the existence of an absolute meaning because meaning is never stable and it is always slippery, still to come, differed. Instead of Postmodernism, Modernism is still looking for a final destination of meanings and an abstract truth of life.
Postmodernism abolishes the difference between low and high art whereas Modernism constructs a coherent and equal world view.
Talking about literary works, in Postmodernism they are products of intertextuality whereas in Modernism they are considered as unique creations of the artists.
The biggest difference between Postmodernism and Modernism is that the first believes in the concept of relativity instead of the second which is based on using rationality.