As Ben says, 'How do you do?' 'How do you do?' is a formal exchange for the first time you meet someone. In fact, it isn't a question at all - it's the equivalent of saying 'I'm pleased to meet you'.

When someone says to you, "How do you do?", it does not mean "How are you?" It just means "Greetings". The proper response to 'How do you do?' IS a reciprocal 'How do you do?', as in this exchange from Oscar Wilde's Lady Windermere's Fan, 1892: Lord Darlington: How do you do, Lady Windermere? Lady Windermere: How do you do, Lord Darlington? The proper response is not "I am fine" or "How are you?" Note that "How do you do?" is only used the first time you have ever met a person. Thereafter, you should use "How are you?" when you meet.

Just keep in mind that "How do you do?" sounds very formal in American English. I've only ever used it (or heard it used) when someone is joking around.

How do you do is defined as a polite greeting that you can use when first introduced to someone. An example of "how do you do" is what you would say as you shake hands with someone you meet for the first time. But I'd rather to say " How are you? " in all situations. ----------------- source : yourdictionary.com & myself