A metaphor is a word that directly links two seemingly unrelated things, like God and a fortress as in "a mighty fortress is our God".
Metaphors are figures of speech used in poetry and various other types of creative writing. They're employed in the same way as similes, but without the words "like" or "as". One of the most famous metaphors might be Shakespeare's "All the world's a stage".
You probably use metaphors yourself all the time without necessarily realizing it. For instance, if you've ever referred to "that neck of the woods" or to someone who's "rolling in dough" then you've been metaphorical.
Another beautiful metaphor can be found in the phrase "the lake mirrored the moon." If you said simply the moon was reflected in the lake, or that the lake was flat and shiny like the surface of a mirror, then the phrase loses some of its power and magic.
Common Link
Metaphors use figurative language: that is, they create an image in which two unrelated items share a common quality:
A heart of stone
Stubborn stains
The chimney belched smoke
Whereas if you said "he smokes like a chimney" that would be a simile.
Typical Usage
Metaphorical language can be found wherever writers make comparisons in an attempt to increase our understanding. Examples can be found in prose, poetry and song lyrics, such as these lines from Leonard Cohen's song Hallelujah:
There's a blaze of light
In every word
Or this fantastic metaphor from the poem Fog by Carl Sandburg:
The fog comes
on little cat feet.
It sits looking
over harbor and city
on silent haunches
and then moves on.
This whole stanza is a metaphor in which fog is given the qualities of a cat, creeping quietly in, surveying the scene and just as silently moving off again. Anyone who's owned a cat will recognize these characteristics of stealth and independence immediately.
And to illustrate the difference one last time, here's a simile from Cohen's song Suzanne:
And the sun pours down like honey
On our lady of the harbour
Literary Examples
Metaphors litter the volumes of poetry loitering on library shelves across the world. Here are some terrific examples from T.S. Eliot's The Love Song of J. Alfred Prufrock:
The yellow fog that rubs its back upon the window-panes,
And time for all the works and days of hands
That lift and drop a question on your plate;
I have measured out my life with coffee spoons;
Or this powerful image from the poem He Wishes for the Cloths of Heaven by W.B. Yeats;
I have spread my dreams under your feet;
Tread softly because you tread on my dreams.
Sometimes the metaphor can be simply put, as Dickens did in A Christmas Carol:
This boy is Ignorance. This girl is Want.
Everyday Use
Metaphors play an important role in our everyday vocabularies, too. For instance, you might say that this article sucks. Or if you enjoyed reading it, you might say that time flies when you're having fun.
After a walk on a cold, snowy winter's day, you might say your feet are blocks of ice. Or that you could murder a cup of hot chocolate.
But unlike a vacuum cleaner, an article can't suck. Time can't fly, and you can't murder a beverage. And that's the power of metaphor: it imparts the qualities of one thing to another, spicing up the language and helping to make the written word more beautiful and dynamic.