I am but a poor speaker; my words stumble from my tongue (or keyboard) like bleary-eyed tosspots after a night's carouse. But I'll do what I can with my poor skills to discuss rhetoric.
"Doesn't everyone know what a rhetorical question is?" I hear you ask. But questions that don't require answers are not what rhetoric is all about. Rhetoric is the art of speaking or writing well. And rhetorical devices are techniques for speaking or writing well; weapons in the arsenal of the speaker or writer, tools in the toolbox.
Longtime readers have seen plenty of rhetorical devices discussed in these columns: syllepsis and zeugma, euphemisms, alliteration, oxymorons and tautologies, and of course plenty of paronomasia, among others. Many other such devices are well known: irony, metaphor, onomatopoeia, and so on. But there are also plenty of rhetorical devices which are commonly practiced but are not commonly referred to by name.
For example, the insertion of a word or phrase into the middle of a word is called tmesis, one of my favorite words. So next time you hear someone use the phrase "a whole nother," or exclaim "fan-fucking-tastic!", you'll know they're engaging in tmesis.
Similarly widely practiced is pleonasm, or (as I noted in a recent column) repetition. For that matter, over-wordiness in general has a name: perissologia.
Another commonly used rhetorical device is understatement for effect, or meiosis. "Yeah, my car got ever-so-slightly dented when I plowed into the tree at sixty miles an hour."
And another: anthimeria, the use of one part of speech for another, including verbing nouns.
One more: aposiopesis, which involves breaking off in the middle of a sentence, as in, "When your father gets home, he'll—"
So why name all of these techniques if everyone knows how to use them? Precisely because rhetoric is an art. To learn such an art, to become a more effective speaker or writer, it helps to have some awareness of the range of possible techniques available for expressing oneself. And to discuss these techniques, it helps to have a commonly agreed-upon vocabulary. The skills of rhetoric have been discussed and written about since the time of the Greeks; there's a vast body of old writings on the topic, as well as plenty of more current discussion of modern rhetoric.
Rhetoric can, of course, produce powerful effects. Some such effects are produced carefully and purposefully by speakers who've been trained in using various techniques. When Winston Churchill said, "We shall fight in France, we shall fight on the seas and oceans, ... we shall fight on the beaches, we shall fight on the landing grounds," and so on, he knew that that repetition (and there are many forms of repetition, each with its own name) would help to make his point.
My favorite rhetorical devices are mostly those in which a speaker claims something to be true while implying the opposite. Sarcasm works that way, of course, but there are subtler forms. For instance, praeteritio, also known as paralipsis: pretending one is omitting information while providing it. "I shall refrain from mentioning my opponent's lengthy criminal record, his penchant for spitting on babies, or the fact that he's nigh-worshipped by neo-Nazis all over the world..." Or for that matter "We hold these truths to be self-evident" followed by a list of said truths. (If they were really self-evident, they wouldn't need to be stated.) When taken to extremes, giving full detail on all the things the speaker claims not to be mentioning, paralipsis becomes known as proslepsis.
I think there's another name for the more general rhetorical device of claiming not to be saying something when you're saying it ("I come to bury Caesar, not to praise him"), but I forget what that name is. And I believe I've encountered a name for the rhetorical device of starting out by claiming not to be an eloquent speaker, but none of the sources I can find provide that name...
If you want to know more about rhetoric, there are several good online resources. Several friends and readers have pointed me to the Rhetoric Handbook, which defines many terms and provides examples. (Danny Fahs also pointed me to a page of Linguistic Phenomena and Devices.) The Rhetorical Figures Glossary page provides links (at the bottom of the page) to several other online sources of information about rhetoric and rhetorical devices, ancient and modern. And Silva Rhetoricæ, The Forest of Rhetoric, provides definitions and discussion of even more rhetorical figures, along with a good introduction to the subject of rhetoric.