The Little Prince is about love. It's about how to take care of the things and the people that are important to you... and which things and people should be important to you. Saint-Exupery conveys his point using characters and plot elements that are simple but not simplistic, dramatic but not melodramatic.
The book centers on the relationship between the author and the prince, a strange little traveler he befriends one day in the desert. These characters are strongly developed and highly sympathetic; because the book is written almost as a memoir, in the first person, readers are swept deep into the author's thoughts and also come to a partial understanding of the prince's mysterious life through his eyes.
Other characters in the book are really more like caricatures. Several adults whom the prince meets during his travels represent unimportant concerns that often distract grown-ups. Others, like the prince's rose and fox who uttered the famous line, "What is essential is invisible to the eye," represent important concerns whose power grown-ups often underestimate.
Readers learn more about the prince and the author through their interactions with and reactions to these one-dimensional personalities.
You learn that the author has lost -- and desperately misses -- his childhood innocence. You see that the prince is in peril of losing his. You watch them rely on each other to maintain an outlook on the world that allows time to draw sheep, rescue flowers, and laugh. And, naturally, you see a little of each character in yourself.
And it's a little upsetting. That's the beauty of the book. Saint-Exupery makes very clear his thoughts on which course is right, who has lost perspective. He makes very clear the notion that, in some ways, we are like the prince, but in many ways, we are like the silly grown-ups the prince meets. He makes us want to reconsider.
Kids will love this book for the fantastic, magical stories it contains, although they may miss its deeper significance. Adults will love it for helping them regain a child's sense of Wonder and Importance. I promised myself that I would read this book once a year until I die. Just so I won't forget that life's joys come from simple things.