1) using a adapter, screw thread 1 side, camera mount on the other side. only usable for slr & dslr.
so you screw this adapter to front of lens like a filter. after that, you reverse mount your lens, ie, rear element of lens now becomes front.
2) using a adapter, male thread to male thread, size of thread can be the same or different. usable on compact digital with lens that can mount filter or via an filter extension tube, as well as slr & dslr.
so you screw on the adapter to the lens of the camera, than you reverse another lens and screw it onto the adapter, which in this case, has a male thread exposed.