有一些english ga problem

i have some problems , hope someone can help me to solve
When u meet a friend on the street, you may either say "how are you" OR "how do you do"
The problem is , what are the differences between "how do you do " and "how are you"
Is there any difference in degree??
Or this two questions are just the same.
and usually how will you answer??

Thank you very much.
Welcome to use chinese to answer my question.