What is the difference between imagery and symbol?

Imagery in literature is used to paint a mental image of
something.  The techniques used are descriptive and paint a picture that allows the
reader to visualize the setting, person, or image that is intended to be
conveyed.

An example of this would be: The old farm
encrusted with barren soil and remnants of long decayed crops stood lonely and isolated
as the wind pounded its walls.

Symbols are when w writer
uses an object or idea to represent something else.  It is usually something that the
author feels is significant in the life of the person or central to the stories
theme. 

A good example of symbol use is found in the book
"Lord of the Flies."  The boys in the story take a pig's head and stick it on a stake
that is pointed at both ends.  Then the head is lodged into the ground.  The head
represents a religious icon as the boys move away from civilized behavior.  It also
represents the savage behaviors that the boys engage in.