The Cleric class has never really made a lot of sense to me. It seems to represent the clergy within a religion, yet it is more capable in a combat role than the monk who specializes in combat. In that regard, the Cleric is more like a cross between a Paladin and Clergy. The Cleric class also suffers from the same problem as the Wizard class with regards to the spell slot system. You know: having to prepare spells before you even know what you need to cast that day.
The Priest class attempts to rectify these things. It uses a spell point system similar to that of the Mage class. It also reduces the combat effectiveness of the character to make him more like a member of the clergy. Spell casting is now much more powerful in that there is no need for memorization. The Priest is more like the extremely studios mage except that he's extremely pious.