Showing Not Telling

One of the 'rules' of writing is to show not tell.
What's the difference between showing and telling?

Telling is when you, the writer, come out and say it.

She was shy.
She was afraid of snakes.
Her hair was brown.

Showing is when through the character's actions, thoughts, and words the reader learns these things.

She blushed and tucked a strand of brown hair behind one ear.
"I'm...scared of snakes," she whispered quietly, nervously scraping her foot against the floor.

Some more examples;

Telling:

Marie hated Daniel.
Marie cried when she got upset.
Marie had blue eyes.

Showing:

Marie ripped herself from Daniel's grasp.
"Don't touch me," she hissed, her blue eyes brimming with angry tears.

Showing is usually more vivid and appealing than Telling.

But Telling has its place too, something people tend to ignore.

You can say "Five months had passed," rather than showing in detail what happened every day during those months.

Telling is also useful when writing a action scene. Some writers shy away from writing fight scenes, because they bog themselves down by worrying about showing.
Most action is done quickly. Simply saying, "Jared slammed his fist into Ben's face," will suffice.
For fast paced scenes, don't worry about showing us the fight. Just get on with it already.

But that's another post.