Ideally, a metaphor has two parts, a tenor and a vehicle. The tenor is the thing being discussed, and the vehicle is the object of comparison. Now, a metaphor usually is considered a direct comparison, i.e., "All men are pigs," while a simile is considered an indirect comparison, "All men are like pigs," and a juxtaposition is a forced comparison, where men and pigs would be placed so close to each other as to draw a comparison. Also, one must consider descriptive metaphors, i.e., "Men root around in the slop of their troughs, waddling on four cloven hooves, their tusks piercing the mud of their styes..." where the description implies a pig without actually saying "pig". But this is the stuff of Intro to Poetry courses, readily available in any textbook.
That being said, how exactly does a metaphor work? Most would argue that the vehicle should expose or open one's perception about the tenor. However, there are those who would argue that the vehicle also serves to limit or focus the tenor, exploring only particular aspects of the tenor which the poet wants to expose.
The questions I pose to readers:
1) What is your favorite metaphor, from poetry, prose, film, speech, music, etc.
2) How do you think that metaphor works?
I'll give an example that makes me seethe in jealousy every time I hear it:
"The Mississippi Delta was shining like a national guitar."
This simile, from Paul Simon's "Graceland" is beautiful on many levels. First off, the tenor is rich with musical implication--blues, rock and roll, jazz, gospel, etc. The fact that the title of this song is "Graceland," with all of it's Elvis connotations, as well as religious connotations, compiles more layers of implication and depth onto the tenor. Then, Simon uses a strong verb--shining (connotations of light, sunrise or sunset, brilliance, etc.)--and a musical vehicle to strengthen and open up those musical and religious connotations. The guitar is an instrument, which is played. What does it mean, therefore, for a guitar to not sing or make music, but to shine? What sort of music carries that connotation of light, and how does it stem from or relate to the Mississippi Delta? Again, both the secular and religious music of the American South is brought into play, as well as the influence of all that music on Elvis Presley. Listening to the full song, one realizes that "Graceland" itself becomes a vehicle, and leaves Memphis and becomes literally a land of grace, and possibly redemption, for the speaker.
So, what is your favorite metaphor and how does it work?