Like wikipedia says: Workbench is the native graphical desktop environment for the Amiga computer. Workbench is not the operating system, it's just a desktop environment running on top of AmigaOS.
Workbench has always been very limited in file handling area and that's why Amiga has many 3rd party (graphical) file manager programs and Directory Opus (DOpus in short) is one of them.
Up to version 4 DOpus used traditional two view layout and it was always ran as a separate program on its own window or screen, but with version 5.x it changed its philosophy completely. DOpus 5.x does much more than those old school filemanagers and it can even be used to replace whole Workbench as desktop environment.
DOpus 5.x is very configurable and flexible program and everyone can config it to one's needs. You can run it as separate program or you can make it look just like Workbench, but a bit more hidden under the hood. And everything between. Especially "power users" like it. And usually other people's configs don't suit to you, so better go through the configs yourself
