In simple terms:
"WHY" is used to ask a question concerning the cause or reason for which something is done.
Eg:
Why did you leave so early?
Why did she cry?
"BECAUSE" answers the question to "WHY". Because means "due to".
Eg:
-- I left early because I was getting late.
--She cried because she was sad.
Why- can also be used as an exclamation of surprise.
Eg: Why, Its not here now!