the difference between modern, pre-modern and post-modern?

Answer 1

Timing. There are style elements that put works of art in a specific era but mainly it is about when they were made.

"Modern art is a general term used for most of the artistic work from the late 19th century until approximately the 1970s."
-Wiki
http://en.wikipedia.org/wiki/Modern_art

Answer 2

More or less clear answer is possible only with regard to architecture: modern started with Bauhaus, Russian constructivism and American architecture of glass and concrete (Ludwig Mies van der Rohe probably the best and most typical). Pre-modern can be named searchings of fin-de-ciecle and early 20th century: Art Nouveau, Art Deco etc. Post-modern has come after the modern to bring some diversity, again experiments with historic motives, play with different patterns, combining what is hardly compatible etc.