The term Postmodernism is used to describe the change that took place in western society and culture from the 1960’s onwards that arose from challenges made to established structures and belief systems.
In art postmodernism was specifically a reaction against modernism which had dominated art theory and practice since the beginning of the twentieth century.
Postmodernism art:
Whaam! – 1963 – Roy Lichenstein

http://www.tate.org.uk/art/artworks/lichtenstein-whaam-t00897
Away from the flock – 1994 – Damien Hirst

http://www.tate.org.uk/art/artworks/hirst-away-from-the-flock-ar00499