How many times have you heard that one? Sometimes a story can be too showy or too telling. I think the best is trying to find the perfect balance between the two. So, what's the difference?
Telling is basically just giving the facts. It tells the story, but doesn't put the reader inside the action or emotions of the scene.
Example: Sitting on the sofa, she looked exhausted.
Showing is describing the scene in a way that brings the reader inside the story. This is where the scenic descriptions and senses (sights, sounds, etc.) come into play.
Example: Her eyes told of her pain; deep, set-back, reaching inside of herself. Dark caves formed where her cheeks were. Her mouth was a hardened straight line, down at the corners.
(These 2 examples were taken from a pretty good writing site--http://www.dvc.edu/english/Learning_Resources/showingVsTelling.htm)
Here's one more example--
Valek was angry. (Telling)
"Valek took a gray rock off his desk and hurled it toward me. Stunned, I froze as the stone whizzed past and exploded on the wall behind me." (Showing)
(These 2 were taken from another great site with some ways to insure you're showing and not telling--http://www.mariavsnyder.com/tips/showvstell.php)
Like I mentioned before, I think the best works are those that have the perfect blend of showing and telling. Here's my own example. Notice the first and last sentences are telling, while the rest of the paragraph is showing.--
Winter was coming. The wind whispered through the tree branches, pulled the remaining leaves from their limbs and sent them to their deaths. Chipmunks, cheeks full and round, squirreled away the last of the nuts. The once bright green blades of grass were wilted with sorrow. Winter was coming.
Showing also goes a long way to set the tone of your chapter. Bright colors and cheerful sounds could be used for a joyful scene, or, even better, they could serve as a slap in the face when something horrible happens all of a sudden.