The generic, non-branded gadget slides into a hot shoe or cold shoe and uses mirrors to bounce the view to the top of the camera, rather than relying on a standard viewfinder or LCD screen.
See DSLR. Small, point-and-shoot cameras use a "live preview" LCD screen to take pictures and display the stored images. Digital SLRs have a traditional viewfinder that you press your eye against ...
While other manufacturers have bolstered their lower end compact system camera offerings ... to include an integral viewfinder alongside a regular LCD. Just like on a DSLR you have a choice ...