Allegories and symbols are closely related, and people often confuse them. A symbol is a a single object, person or idea that represents something else. For example, in C.S. Lewis' "The Lion, the Witch and the Wardrobe," the character Aslan is a symbol representing Jesus. The defining characteristic of an allegory is that it is composed of a set of connected symbols. In "The Lion, the Witch and the Wardrobe," Aslan is not the only symbol -- many of the characters and places in the story symbolize people and events from the Bible. Therefore, in part, the book is an allegorical retelling of the story of Jesus.
One of the most common forms of allegory is allegorical personification, in which a nonhuman concept, such as virtue, faith or sin, is represented as a person in a story. These characters have no personality traits independent of the concept they represent. For example, in Nathaniel Hawthorne's "Young Goodman Brown," the title character's wife represents the concept of faith and has no personality beyond that. Hawthorne even names her Faith, a common technique in allegorical personification. In a literal reading of the story, Faith is Goodman Brown's wife, and his love for her makes him rethink his plan to attend the witches' Sabbat; in an allegorical reading, his faith in God and adherence to church doctrine make him question his decision to give himself to the Devil.
Allegories typically include a moral, or a lesson for the reader. Most often, these morals are ethical or religious, as in "Young Goodman Brown" and "The Lion, the Witch and the Wardrobe." "Aesop's Fables" is another famous example of allegories with morals. Each fable features characters who represent decisions or personality traits, and the stories teach lessons about the effects of these decisions or traits on people's lives. They are unusual in that their morals are explicitly stated at the end of each fable.
Allegories that do not have morals are often political satires, in which characters and events represent political parties, movements or ideologies. For example, George Orwell's "Animal Farm" is an allegorical satire. The animals represent the players and ideologies that fueled the Russian Revolution. While allegorical satires do not have morals in the traditional sense, the writer does use the story to persuade his readers to share his opinions. For example, Orwell does not use "Animal Farm" to teach a simple moral like "slow and steady wins the race," but he does want his readers to understand why the Russian Revolution hurt Russia more than it helped the country.