Symbolism is the representation of something abstract with something concrete. For example, a peace sign is a symbol. It's something concrete (meaning that it can be detected by the senses; in this case it can be seen). It represents or symbolizes an abstract idea--the idea of peace. Peace is not something you can detect with the senses, although you can detect evidence of it. In a similar way, the dollar sign is something concrete that symbolizes the abstract idea of wealth.
In literature, writers can create symbols through language. In Lord of the Flies, the pig's head can be said to symbolize or represent the danger of mindless superstition. The pig's head is concrete, mindless superstition is abstract.
Imagery, on the other hand, is the use of words that appeal to the senses. When a writer uses imagery, he is usually describing something rather than trying to create symbolic meaning. Look at the following line from Wordsworth's poem "I Wandered Lonely as a Cloud."
"The waves beside them danced; but they outdid the sparkling waves in glee."
Here, the word danced creates an "image" for the reader--we can see the moving waves in our mind. "Sparkling" creates another image, and we can see the sun glinting off the water. Thus, by using words that appeal to the senses, the poet has created imagery that makes the poem something closer to a real experience.
Keep in mind that writers also use words that enable readers to engage their other senses, it doesn't have to be sight.The waves beside them danced; but they
Out-did the sparkling waves in glee: