I know a bit, not much.
The basic Opinels with which many are acquainted are French made. They have, I believe, pearwood handles. They are folders. The blade is opened by pulling on a nail nick. There is a metal ferrule at the pivot end of the handle. This ferrule has a slit in it, which the blade moves. The ferrule can be twisted to either side, thereby locking the blade very simply and solidly.
Most of the Opinels are made with carbon steel blades. A couple, the model 64 and 84, I think, are made with stainless steel. They are quite inexpensive usually. In fact, their price in brick and mortar stores is the closest to that of web dealers of any knives I've seen. This makes them ideal conscience salvers. You can handle all the knives that you're interested in buying, then buy an Opinel. The merchant makes a couple bucks and you don't feel so guilty about not buying anything from him.
Opinel makes much fancier horn-handled knives, which are quite a bit more expensive than the common wood-handled knives that are so well known.
They are basically very good knives, especially for the price. As someone pointed out in just the last couple days, we could all probably get by with Opinels, and don't need all the fancy knives we hold so dear.