In British English, `How do you do?’ is normally used when you are introduced to someone for the first time. It is a way of greeting someone, and the standard response to this expression is, `How do you do?’ You normally use this expression only once with a person. The next time you meet him, you can say, `How are you?’ This expression is used with people you already know. Unlike `How do you do’, `How are you’ is intended to find out how the individual is doing — both emotionally and physically. `How do you do?’ does not carry this meaning.
Did you know that the word `hi’ is actually from `how are you’? When `how are you’ is said very fast, it sounds like `hiya’. So instead of saying, `how are you’, people started saying, `hiya’. Later on, `hiya’ was reduced to `hi’. Therefore when you meet someone and say, `Hi, how are you?’, what you are actually saying is, `How are you? How are you?’.