Postmodernism refers to a socio-cultural and literary theory, spreading it's characteristics to science, architecture, literature, art, etc. It's general idea, in simple words, was that everything had already been done, therefore an original idea or thought couldn't be produced. Postmodern authors then used this lack of originality as a start point experimenting with classical elements, mixing low art with high art, the past with the future. They wanted to test the boundaries of what could and couldn't be done, making them the rebels of their time (1950's).
Here are some postmodern authors: Kurt Vonnegut, Samuel Beckett, Jorge Luis Borges, Angela Carter.