What is the difference between following two statements?
- How are you doing?
- How do you do?
Both are stock, polite greetings. They both REALLY mean "hello", simply acknowledging the other person's presence. The stock response is "Fine, thank you, and you?"
It's very common (and thus boring) for people to point out that no one expects a real answer to the question. If you said to some one, "How are you doing?" and they proceeded to describe their health or social problems, it would just be strange. It would be even stranger if they actually described how they were doing their current task. Like, "How do you do?" "Oh, I do this by inserting the plugh into the foobar, wrapping the nanobucket around the widget, and then ..."
In short, neither sentence really "means" anything. They're just polite stock phrases.