A rhetorical question implies its own answer; it’s a way of making a point. Examples: “Aren’t you ashamed of yourself?” “What business is it of yours?” “How did that idiot ever get elected?” “What is so rare as a day in June?” These aren’t questions in the usual sense, but statements in the form of a question.
Many people mistakenly suppose that any nonsensical question, or one which cannot be answered, can be called a rhetorical question. The following are not proper rhetorical questions: “What was the best thing before sliced bread?” “If a tree falls in the forest and no one hears it, does it make a sound?” “Who let the dogs out?”
Sometimes speakers ask questions so they can then proceed to answer them: “Do we have enough troops to win the war? It all depends on how you define victory.” The speaker is engaging in rhetoric, but the question asked is not a rhetorical question in the technical sense. Instead this is a mock-dialogue, with the speaker taking both roles.
How about words that change meaning? Paul Brians' latest blog post looks into that phenomenon.