A symbol in literature is when an author uses an object, an event, almost anything really, to represent an idea. A metaphor is an implied, or indirect, comparison. A metaphor can be used in a symbolic way in literature.
In "Fahrenheit 451", Bradbury has some metaphors that are symbolic. The seashells, for example, are metaphors for earbud radio speakers. They aren't really seashells, they just look like that so they are called seashells. That makes the name "seashell" a metaphor. The symbolism there is that in the society of the novel, people wear the seashells indicating that they are connected to some disembodied voice on the radio and disconnected from the people around them. One of the ideas Bradbury brought out in the book was that people were becoming too connected to technology and too disconnected with people.
The firetrucks, in the book, were called "Salamanders". Calling them that is using the word "salamander" as a metaphor for the vehicle. The symbolic purpose of the word relates back to the mythological belief that a salamander could walk through fire unscathed.
Calling the book people, encountered at the end of the book, by the books each has memorized is metaphoric; i.e., calling a person "David Copperfield" because he has memorized that book is an implied comparison of the person to the novel of that name. Symbolically, "David Copperfield", the metaphor, represents the hope that no matter how bad the world may become, there are those who will resist and hang on to the great works of literature.