Here are some examples of prominent post-war novels:
The Catcher in the Rye
Most simply, a postwar novel is any novel written after a war. Since World War II is the major war of the twentieth century, that phrase is most commonly used to mean a novel published after World War II. As far as definitions, that's it. There are, however, further associations with it. Postwar novels tend to focus on the state of Western /industrialized society in the modern age. They tend to deal with alienation, with urbanization, with change, especially political change, and with human response to it.
You'll find a link to a syllabus listing representative postwar novels below.
In addition to the first answer: A post-war novel frequently reflects on how the war altered the economy, the society, the psyche, the everything immediately thereafter the war or over time after the war.