Show don’t tell. It’s one of the oldest axioms in the creative writing world. The example I always use with my students is:
Mrs. McGillicutty was mean.
Mrs. McGillicutty smiled as her rocker rolled over the cat’s tail and it let out a tremendous yowl.
Basic, but you get the idea. However, there are times when it’s important to tell and not show. I’ve been thinking about this as I revise my YA novel. Perceptive agent pointed out that there’s quite a bit of what she called “walking around music” in my current draft. By this she meant that I’m describing a scene in detail that I could simply summarize in a few quick lines. The scene itself isn’t actually all that important for the reader to witness first hand. For example, in one place my main character convinces her parents to let her go to the bonfires. It’s not actually all that important how she convinces them, so instead of writing out that whole scene I can simply sum it up and the outcome is the same and the story line holds more tension.
A more obvious example of this would be if you described a character tying his or her shoes instead of simply saying he or she tied her shoes.
Isabelle carefully laid one lace over the other, making a loop and then chasing the rabbit through the hole as she had learned in Mrs. Wilcox’s warm sunny kindergarten room. She gently pulled the loop through and…
Isabelle tied her shoes.
Now unless Isabelle is recovering from a traumatic head injury, the first description is really unnecessary. I haven’t unearthed any examples in my own writing that are quite this flagrant, but it has helped me trim quite a bit of unnecessary fat from the manuscript.