A Christmas Carol is a short novel that at its core, presents the concept of personal redemption through selflessness. The moral lesson within comes from the contrast between the rich and the poor. Dickens does not simply portray the difference between being cold and hungry versus being rich and warm, but he shows the difference in attitudes of the "haves" and the "have nots."
Bob Cratchit is a man who is materially poor, but rich in love, rich in hope, rich in family support, and even rich in forgiveness and selflessness. His cheerful attitude and willingness to share even the little that he has shows that in his heart, he is a righteous man.
Scrooge, on the other hand, is wealthy, but lonely, angry, selfish, and living in constant fear. He has nothing but enemies. He trusts no one.
These two characters are the first thing that make this story so entertaining. They are both very different, yet equally likeable characters. They have been portrayed in several films and stage productions, and most audiences agree that though dispicable, Scrooge is still an excellent character. And of course, no one can deny both pity and love for Cratchit (or Tiny Tim).
Another reason this story is so entertaining is the originality of the storyline, and the presence of ghosts. It is an adventure, and the entire adventure takes place in one night. Scrooge's life flashes before his eyes in such a dark but realistic way, that the audience can't help but put themselves in his shoes. We are meant to wonder, if we were taken on a similar journey of our own lives, what would we see? And what would we want to change as a result?
This is a classic tale of redemption, forgiveness, and ultimately triumph. Audiences are entertained because the characters are likeable, and the story itself is as realistic as it is fanciful. Yet, it is also a tale of morality, and the truths it holds are universal and immediately applicable.