It is my understanding that the belt holster is designed to be held onto the pants/body by threading the belt through loops in the holster (by pants I am referring to slacks, jeans, et cetera), and both gun and holster ride outside of the pants, whereas a paddle holster is not physically attached to the belt, but rather held onto the body by means of a "paddle" being placed between the body and pants. (inside of the pants). The holster and gun ride outside of the pants, being held in place by the pressure exerted upon the paddle (which is attached to the holster, then slipped inside of the pants) between the body and pants.
Basically, KBOYCI is correct... The Paddle makes it more convienent for being able to remove the holster without unthreading the belt, but often does not provide the same level of security or stability. It depends on the design... I have a couple of Safarilands paddles which are very good.
Here's a couple pics to help.
This holster can be fitted with a Belt loop accessory that replaces the paddle so that it can be threaded onto a belt...