What does it look like, sound like, feel like, taste like, smell like? When you describe a person or event, your reader is there with you. When you tell,the reader relaxes to the point of mental slumber.
Not sure of the difference?
Telling: John was sad after Susan broke up with him.
Showing: John shut his cell phone and leaned against the wall. He heaved a sigh and dropped his head into his hands.
Hear the reader's mind working:
"What's with John? Oh, I get it, he feels Susan let him down."
In nonfiction, details show, generalities or opinions tell.
Telling: Children are out of shape these days.
Reader: "I don't think that's true. My neighbor's kid plays Little League."
Showing: Forty percent of 5 to 8-year olds are obese.
The reader's mind kicks in:
"Wow! Children are out of shape these days!"
Click the picture, it leads to were I got this information