Warning: Wikipedia contains spoilers
The theme of Atlas Shrugged is the role of the mind in life and society. Rand argues that independent thinking, and the creativity and inventiveness that comes from this, is the motor that runs the world. In Atlas Shrugged she shows what she thinks would happen to the world if the "men of the mind" went on strike: the motor of the world would shut down, and civilization would fall apart. This is a direct assault on the Labor Theory Of Value, which was popular at the time the novel was written.
Rand suggests that a society will stagnate to the extent that independence and individual achievement are discouraged or demonized. Inversely, a society will become more prosperous as it allows, encourages, and rewards independence and individual achievement. Rand believed that independence flourishes to the extent that people are free, and that achievement is most highly rewarded where people are allowed to keep the product of their efforts. She advocated laissez-faire capitalism as the political system most consistent with these beliefs. These considerations make Atlas Shrugged a highly political book.
Rand also argues that traits like independence and individual achievement which drive the world are actually virtues, and in her worldview are central to a "rational" moral code. She disputes the notion of self-sacrifice as a virtue. These ethical considerations are prominent in Atlas Shrugged.
According to a joint survey [1] conducted in 1991 by the Library of Congress and the Book of the Month Club, Atlas Shrugged is recognised by numerous authors as the "second most influential book for Americans today", after the Bible. In addition, the Boston Public Library has named Atlas Shrugged as one of the most influential books of the twentieth century. [1].
Exactly when Atlas Shrugged is meant to take place is kept deliberately vague. In section 152 the population of New York is given as 7 million. The historical New York City reached 7 million people in the 1930s, placing the novel sometime after that. There are numerous early 20th century technologies available, but the political situtation is clearly different from actual history. It is as if history had changed around 1900, and the world went unimpeded down a gradual path towards socialism for perhaps 40 years, with no World War, and no rise of fascism in Europe. There are many examples of early 20th century technology in Atlas Shrugged, but no post-war technologies such as jet planes, nuclear weapons, helicopters, or computers; television is a novelty that has yet to assume any cultural significance, while radio broadcasts are prominent.
Most of the action in Atlas Shrugged takes place in the United States.
A synopsis of Atlas Shrugged is available on Wikibooks.
Setting
Note: a consolidation of these numerous articles is in progress.