In the simplest terms, this is how I think of them:
The social sciences search for truth.
The humanities and arts search for understanding.
One asks what questions, the other why questions.
You look at PIcasso's Guernica to understand horror.
You study data to find out who travels to museums to see Guernica.
Now you will get arguments about this, of course, from positivist social scientists, interpretists, postmodernists, artist, narrative writers, musicians, etc., etc.
I personally make no distinction between the expressions human sciences (or humanities if you will) and social sciences. Prof. Ivo Carneiro de Sousa's answer is quite enlightening for those wishing to understand why both expressions co-exist. But in practical terms, I agree that a distinction would be based upon quite artificial boundaries...
For example, I attended a Liberal Arts College as an undergraduate. There one could obtain degrees in either the arts or the sciences depending on the precise subject area to be on the diploma.
However, I should add that if one received a bachelor in the sciences was also sometimes a personal choice. For example, if a student took a degree in sciences, he/she could add more liberal arts coursework and graduate with an arts diploma in certain cases.
Meanwhile, we have the division between the humanities which may include the social sciences while not including the so-called hard, technical or natural sciences.