What is the difference between a "god" and a non-god power in the Forgotten Realms?
I assume that a "god" can grant cleric spells and that a non-god power cannot.
I am assuming D&D 5th edition.
Godhood is decided primarily based on power, which in turn is generally decided based on the number of worshipers the prospective deity has (paraphrased from DMG 5e, p 11 under Divine Rank).
That means that gods - both lesser and greater - grant spells to clerics as a defining trait. Not being able to grant spells to clerics makes immensely powerful beings quasi-dieties of some sort.