What is the difference between modern and postmodern art?
Generally, modern art is seen as that which began around the time of the first Impressionist exhibit in the late 1800's at the Salon in France, where, of course, they were ridiculed. The name "impressionist" was actually intended to be an insult by one critic discussing what he claimed were random brushstrokes with no particular direction or purpose. This was a radical departure from the still lifes, portraits, landscapes and historical paintings of the seventeenth and early eighteenth centuries, which were generally governed by fairly narrow, academic boundaries of what was thought to be "art". Claude Monet did the traditional landscapes, but in a decidedly non-traditional (and to some, horrifying) way, walking around outside observing and painting things like haystacks in varying dawn, daytime, and evening light. Pierre Auguste Renoir and Eduard Manet, to name a few, painted scenes of modern French life, and art became more about personal expression, and less about conforming to academic guidelines and strictures. Postmodern art, in contrast, became less about personal expression and more about the abstract and impersonal (Andy Warhol, Lawrence Weiner) and is generally dated to the mid-1900's.
Visual art as well as music, literature and all other sibling artforms tend to follow the philosophical thinking of the day (or in many cases, push that thinking to the forefront). The differences between Modernism and Post-modernism as I understand them have to do with universal truth verses personal truth. Modernism came on the heels of the Industrial Revolution where science, progress and rationality were thought to be the answers to everything that ailed the human race. Early Modernist painters like Mondrian believed that if they could create a beautiful, balanced work of art, that it would inspire positive change in mankind and promote peace. This seems to be connected to the idea that we as humans are knowable, understandable, and changable. Postmodernism seems to reject this notion under the belief that one person can never truly or fully understand the experience of another human being. From this you have work from artists dealing with their own specific experiences of being in the world (Feminist art, African-American art, etc.). With postmodernism, there is a questioning of traditional structure. An excellent example of this would be to compare the architecture of Frank Llyod Wright, who was part of an era where "form followed function" and contemporary architect Frank Gehry who creates free-form structures that aestectically unique but not always functional in the traditional sense.