paddle holster has, as has been pointed out, a wide folded section at the back that looks like a paddle. the main advantage is that unlike belt mounted holsters you can mount and detach a paddle holster without taking the belt off, making it much easier if somehow you are required to take it off and put it back on in a hurry.
how they won't slide off:
on my paddle holster, there are two protrusions on the inner side of the paddle that cannot be seen from the outside but you can see them when you look at it side ways. they essentially act as a blocker that holds the holster down by going under the underside of the belt from the inside of the pants. it's a little hard to explain in words but it is a very simple concept and one look or try on one and you'll see.
most any gunshows or gunshop will sell the Fobus brand of holster. Folbus makes both belt holster and paddle holster, give it a try and it will become apparent.
pm me if this confused you more than before.