I may be the last person to have finally gotten around to reading this book, and surely I'm the only person who hasn't seen the movie. The plot has been so widely discussed I suppose there's no need for me to describe it.
The surface story level can be called science fiction, but I'm pretty sure that fans of science fiction don't care for the book. And those who like the book are people who wouldn't normally not read science fiction. The book contains an exploration of interpersonal and group relationships that is not usually found in science fiction. The plot is not realistic in a number of ways. However, the first person narrative makes the plot believable by telling it as a child growing up and not quite understanding her environment relative to the outside world. So the reader suspects the presence of an ominous threat behind the obscure descriptions of the planned roles in life. The reader's sole source of information is the voice of the clueless narrator so the reader can be no wiser than the narrator. By the end of the book, and upon reflection, the book's apparent strength is in its ability to make the reader wonder what its second level of meaning might be.
In my opinion it is a parable about human life. The characters live the role that has been laid out for them in much the same way that we all do the best we can with what opportunities we've been given by our circumstances of birth. The characters in this book are ordained to not live a long life for the benefit of others. This has its parallels in the economic, class and racial divisions we encounter in real life. The contrasts in opportunities become even more obvious if considering global and international differences. In real life some die young and others live long due to hidden forces that are stronger than any one individual or group of people. Maybe it's the forces of society, maybe it's God, but fate is not fair.
Some readers wonder why the characters in this book are so docile about their fate. Don't we all die? And we hardly have a choice other than to accept that fact in the end. The best we can do is live our lives to the best of our abilities. Those who protest this reality end up either in jail or a mental institution.