The difference lies not in its seriousness but in its length and structure. A symbol can be any thing – an image, a word picture, a reference to a parallel of any kind – it simply “stands for” an abstract or implied concept: a heart is a “symbol” of love; a flag is a “symbol” of a country. But an allegory is a story or a set of resemblances that show a parallel construction to the abstract concept being relayed: the story of Jack and the Beanstalk is an allegory of the growing from childhood to adulthood – just as Jack disobeys his mother and comes into trouble and adventures, so every child who leaves home will find a life filled with danger, magic, reward, other-worldly creatures, etc. Perhaps the story of Everyman is the most popular literary allegory, in which a physical journey to the grave is an allegory of our life’s “journey.” Arguably all children’s classics are allegories to teach lessons. So, put simply, a symbol is a single representative, while an allegory is a complex set of actions that represent a complex abstraction.
Allegory is a story, poem, or picture that can be interpreted to reveal a hidden message or meaning, it is typically a moral or political one. Symbolism is the use of symbols to represent ideas or qualities. A symbol can be anything, an object or even a person but allegory is a story, poem or picture.