It depicts the transformation into savagery of a group of schoolboys stranded on a deserted island without adult supervision in the aftermath of a plane crash that they got into while fleeing wartime Britain.
It has been said that the author's view on society is such that civilization is merely a thin layer, and that we are really all savages underneath. If the checks and balances of civilization fall away, the 'real', savage nature of humans surfaces. One can see such examples in the real world, e.g. Cultural Revolution in China or mob behaviour during the French Revolution. This is illustrated in the book, as once arriving on the island most of the boys quickly lose their civilised behaviour and form tribal groups. Ralph, the hero, remains to the end of the book the only boy to have retained his original behaviour who is still alive. Piggy is knocked off a cliff by Roger towards the end of the book. His conch shatters on contact with the ground, while the boy's head shatters as it hits the white rock. The body twitches as it slips into the water. Sam and Eric are captured by Jack Merridew, leader of the savages, and are forced to reveal Ralph 's location.
Because Ralph is still alive, becomes hunted by the rest of the group, a tribe led by Jack Merridew. Throughout the book, Golding had each character represent a different aspect of civilisation. Ralph can be thought of as representing democracy - initially he acts by taking votes - whereas Jack depicts savagery. Another central character nicknamed Piggy (we never learn his real name) probably represents science and knowledge, because of his clear methodical thinking. The fact that Roger kills Piggy and Jack hunts Ralph could be seen as Jack's and Roger's (and implicitly the savage human nature's) rejection of civilised society.
For a similar novel, see The Beach.