The theory of signs, technically known as "semiotics," dates back to ancient Mesopotamia and is closely related to theories of interpretation, known formally as "hermeneutics."
The simplest definition of a sign is something that indicates or refers to something else. The earliest study of signs was directed to "natural signs," which are interpreted in two ways. The first was causal. Two examples we find in Aristotle are "If there is smoke, there has been fire" and "If a woman is lactating, she has been with child." In these cases, a sign points backwards to the thing that caused it.
A second type of sign was divine. In Mesopotamia, for example, signs such as eclipses, the flight patterns of birds, or the shapes of the livers of sacrificed animals were thought to be signs of the gods' will. We also see this understanding of signs in Homer and ancient Greek tragedy.
A symbol is an artificial or arbitrary type of sign. For example, there is no reason that the sound "cat" should represent a small domestic feline or that a dove should signify peace. Language is a symbolic system. In literature, one often encounters symbols whose meaning depends either on cultural context (e.g. the Christian symbolism in Bunyan or Milton) or on some system of the writer's own creation (e.g. in Blake or Yeats).
A symbol is a physical object that stands for something else. An example of this could be something like the sun that stands for joy, or clouds that stand for trouble and sadness. Symbols are often used in literature, and especially poetry, to express what the author or poet wants the reader to see or feel. In this way, it is often used to help the reader experience the author’s emotion or thoughts.
Signs are more closely connected to the idea that they promote. It could be a gesture or word that directly state what might be expected. A thumbs-up sign, for example, is the universal sign that things are alright. Signs can also be words or names to provide visitors with information about shops or merchandise. Commonly, these types of signs can be street or shop names.
For a symbol, it is therefore necessary to think a little bit about the connection between the object and its meaning. This meaning is often fluid. Signs are generally less fluid and more likely to have a clearer and concrete meaning.