Novelist Robert Newton Peck explains "show versus tell" in his Secrets of Successful Fiction:
Readers want a picture---something to see, not just a paragraph to read. A picture made out of words. That's what makes a pro out of an amateur. An amateur writer tells a story. A pro shows the story, creates a picture to look at instead of just words to read. A good author writes with a camera, not with a pen.
The amateur writes: "Bill was nervous."
The pro writes: "Bill sat in a dentist's waiting room, peeling the skin at the edge of his thumb, until the raw, red flesh began to show. Biting the torn cuticle, he ripped it away, and sucked at the warm sweetness of his own blood."