A deity in D&D is a being of tremendous power, typically a resident of an outer plane. Clerics derive their powers from their deities, although NWN does not require a cleric to explicitly declare a deity. In fact, the deity field available at character creation has no effect on the default game. (Custom modules can script special effects, though.)
In the Forgotten Realms, deities have particular portfolios (specialties) over which they rule. These portfolios are not permanently assigned, and can be exchanged between deities. (Such exchanges are rarely voluntary by the deity losing a portfolio.) Furthermore, the deities themselves are not permanent, for they can die from lack of worshippers, and new deities can be appointed by Ao.