What type of book is Lord of the Flies considered?

Lord of the Flies is considered to be
a fictional allegory.  An allegory is a work of literature
that functions entirely as a symbol for some other idea.  The characters, setting, and
other details in the work are all symbols that function together to develop another
idea. 

Critics have suggested that Lord of the
Flies may be read as an allegory in political, social, religious, and
psychological terms.  From the political perspective, the
novel represents the conflicts between countries and militaries during the second world
war.  On a social level, the novel represents societal and
governmental hierachies and the power struggles that are inherent in such hierarchies. 
From a religious perspective, the novel may be an allegory
for the Garden of Eden.  Finally, from a psychological
perspective, the novel represents the overall human struggle with inherent evil and
desire.