Clicker training involves shaping behavior in small steps, identifying the behavior, as it occurs, with some kind of marker signal. Dolphin trainers use a whistle; dog and horse trainers have settled on the clicker. But couldn't you just use a word, like "good," or "yes," as a marker signal? And wouldn't it be just as effective?
You can use a word—obedience instructors like the word "yes"—and it will work a lot better than treats alone; but it's not nearly as effective as a click. The evidence from dog training schools that have tried both methods suggests that dogs and their owners learn about 50% more rapidly when the marker signal is a click instead of the word "yes."
The click is easy to hear; words are not. The click is consistent. Words vary from moment to moment and person to person, but the click never changes. The timing of the click is easy to recognize; even beginners can tell if they clicked during the behavior they wanted, or a little too late. But we can't seem to make that same distinction with a word. Maybe clicker classes go faster mainly because people's timing improves rapidly. People who are using a word just don't have the same chance to develop good timing.
Finally, the word "yes" conveys a sense of social approval, not just to the dog but to the person saying "yes." What's the harm, if you are expressing positive emotion? Here's the problem: using a clicker, if you don't get what you had in mind, you just look for the next opportunity to click. Using a word, however, when you can't say "yes" you may feel frustrated and disappointed, and your posture may actually say "no!" The dog feels punished—and immediately the learning slows down or stops. Saving social praise for social interactions, and using a clear-cut mechanical marker signal that means only "you win!" to the dog, can speed up the learning and, strangely enough, remove stress and make the experience more fun for dog and owner, too.