Originally Posted by http://wiki.answers.com/Q/What_is_a_rhetorical_question
What makes a question rhetorical is that it is not asked as a question, but more as a fact or an expression, and does not require an answer.
The rhetorical question is usually defined as any question asked for a purpose other than to obtain the information the question asks. For example, "Why are you so stupid?" is likely to be a statement regarding one's opinion of the person addressed rather than a genuine request to know. Similarly, when someone responds to a tragic event by saying, "Why me, God?!" it is more likely to be an accusation or an expression of feeling than a realistic request for information.
A rhetorical question is a question asked which is intended to influence your opinion rather than require an answer. eg How much longer do we have to suffer this government? Obviously the questioner is not happy with the government and wants you to believe the same: he is not interested in a factual answer.
A rhetorical question is actually a statement that's been phrased as a question to give it more impact. So, instead of saying "I am the greatest dancer in the world!", which sounds a little cheeky, one might say "Who is the greatest dancer in the world? Me!" which sounds more light-hearted, and encourages other people to agree.
A rhetorical question is a question asked without expecting an answer. The question itself is designed to convince the listener.