Realism was a major movement of mid- to late-19th century art, literature, and architecture, and has left a lasting impact on the culture of the 20th and 21st centuries. Before realism, most painting dealt with either historical or allegorical subjects, but afterwards, almost all art has primarily been concerned with contemporary subjects, and allegory is close to unheard of. As Nochlin shows, Realism is not merely a mimetic recreation of what one sees or photo-realism. The Dutch masters and especially Vermeer had produced paintings of great verisimilitude to real life, but they have little in common with 19th century Realism because of the overall social context.
Realism has the unusual distinction of both being a school and not being one. Although many painters could be said to be realists, there is not an easily identifiable school of realist painters like with the Impressionists, who were, in fact, some of the leading exponents of realist principles. There were self-consciously realist writers, like Zola, but as a group they did not actually attach themselves to the label as long as one might suppose.
Nochlin does a great job of explaining precisely what was unique about realism by examining the way that several painters--in particular Courbet, Manet, Monet, Degas, Renoir, and a few other key figures--expressed realist values in their paintings. Her strategy is to do this by discussing realism in the context of several themes. First, she surveys the ways in which realist painters dealt with death. This primarily consisted in the demystification of death, stripping it of the metaphysical, mythological, moralistic, and religious trappings with which it had traditionally been treated. Second, she deals with the question of contemporaneity, which in many ways has been the most influential aspect of realism. As mentioned above, after realism historical painting became increasingly suspect. Finally, she discusses what she terms "The Heroism of Everyday Life." This is the celebration and elevation of everyday activities and people as acceptable and laudable subjects of paintings. The book ends with a Epilogue that takes up the question of realism in architecture, and what this could mean in the context of the 19th century.
Some understanding of realism is essential for understanding both 19th century art, and the development of post-impressionist and abstract art (since these had very different understandings of what was "real"). Like everything that Linda Nochlin writes, this is of the utmost interest not merely to students of art, but to anyone interested in the aesthetic values that went into the making of modernism. I would very much like to see a new edition of this book come out, hopefully with updated illustrations and graphics. It is hard to imagine a stronger introductory treatment of 19th century Realism than this excellent volume.