Warning: Wikipedia contains spoilers.
In this novel, Isaac Asimov first introduced Elijah Baley and R. Daneel Olivaw which would later become his, and more so his readers', favourite protagonists. It is set in a near future where hyperspace travel has been discovered, and a few worlds relatively close to earth have been colonised -- fifty planets known as the "Spacer Worlds". The spacer worlds are rich, have low population density (average population of one hundred million each) and use robot labour very heavily. Meanwhile, Earth is overpopulated (with a total population of 8 billion) and strict rules against robots have been passed. Caves of Steel is a nickname for the huge city complexes in which the Earth's vast population resides. Asimov imagines the present day's underground transit connected to malls and apartment blocks, extended to a point where no-one ever exits to the outside world. Indeed, most of the population cannot leave, as they suffer extreme agoraphobia if they try.
A Spacer ambassador, who tries to convince the Earth government about both the importance of robots and of that of continuing space exploration and colonization, is murdered. Elijah, a detective for the earth police, is charged with finding the murderer. However, he gets a spacer partner -- a humanoid robotic spacer partner named R. Daneel Olivaw. Together, they search for the murderer and try to stop an interplanetary diplomatic incident which could mean the destruction of earth.
In The Caves of Steel and its sequels, Asimov paints a grim situation of an Earth which has become dictatorial/pseudo-socialist to deal with an extremely large population, and of luxury-seeking Spacers who limit birth so that each may have great wealth and privacy. However, the lack of daylight was not something Asimov found grim: one of his anecdotes tells how a reader asked him how he could have imagined such an existence with no daylight. He relates that it had not struck him till then that living perpetually indoors might be construed as unpleasant.
The most interesting aspect of the book is the contrast between Elijah, the human detective, and Daneel, the humanoid robot. Asimov uses the "mechanical" robot thoughts to talk about human nature.
In Asimov's later books, the character Elijah will come to be remembered, thousands of years into the future, as the man who started the second wave of migration from Earth. R. Daneel Olivaw will have a long life, and will maneuver behind the scenes to help humanity as it expand into the galaxy; he will even set the stage for the growth of Galaxia, a living, united galaxy. (See Foundation and Earth for further detail)
Isaac Asimov said that the reason Daneel appeared so often in his books was that his readers and publishers begged it of him.