A Douk-douk is a very simple spring knife made by Cognet in France. It's a plain carbon steel blade, a piece of blued steel sheet for the handle folded over a wire spring.
Wikipedia has
an article about it, or just google for it.
It's a very slim and light knife, the handle doesn't make it suitable for long or hard use (such as carving), but it is a great little knife to keep in an emergency kit, in a desk drawer, or a jacket pocket.