'Not good' is part of a simple negative sentence eg It's not good.
The fixed expression 'no good' refers to something which is completely useless or worthless. If something is completely broken, it's 'no good'. An instruction manual which is full of mistakes is no good to anybody. You can also use 'no good' to mean pointless : for example, you might say 'It's no good asking John for money - he never gives money to anyone.'