The Modern Era refers to 'Modernism', the name given to a set of cultural changes from the late 19th to the mid 20th century, so it includes things such as abstract art, stream of consciousness writing, and developments in psychology. 'Post-Modernism' refers to from the mid 20th century to today, and is characterised by the deconstruction of barriers in arts, so to speak- basically you don't need to follow the set rules any more. Now this is a pretty crappy response I admit- if you head over to /r/askscience and ask a question RE the social sciences, you should find someone to give you a more developed example. Hope this helps!