A symbol is an object, action, or gesture that represents something else. It differs from an allegorical sign in that it has a real, not an abitrary, existence. A rose symbolizes beauty; a goat symbolizes lust; raised arms denote surrender; a cleched fist symbolizes aggression.
Imagery is the use of language to represent objects, feelings, thoughts, actions, ideas, or experiences.
An example of a literary symbol is a journey to the underworld and return (Virgil, Dante, Joyce). Such a journey may be an interpretation of a spiritual experience or a redemptive odyssey.
NOT WANTED ON THE JOURNEY I remember as being a pretty darn good book; I read it about 15 years ago, though, so I can’t remember all that much about it aside from its being a takeoff on the Biblical tale of Noah. But it shouldn’t be hard for you to dig out some symbols and identify them.
[Thanks to the PENGUIN DICTIONARY OF LITERARY TERMS, from which much of the above was paraphrased.]