Here is an image of how it looks on a Leica M6 (rangefinder camera).
The framelines are always lighter than the rest and is used to compose the image (everything within will be part of the image, everything outside will not). The grey area in the middle is used for focusing. In this area you will see two overlapping images, one is part of the whole viewfinder image, and one is projected on top of it. The image on top moves from left to right when you focus. When the two images overlap the focus is correct.
and here is an image showing how the camera looks outside: http://www.adorama.com/images/Product/LCM772BK.JPG
The window to the right is the viewfinder image.
The middle window (grey) is used to make the frames lighter.
The left window is the small focusing area.
The distance between the left and the right window determines the accuracy of the rangefinder. Larger distance = better accuracy.
Here is an image showing how it works:
The blue line is the whole image.
The yellow light is illuminates the frames so they will always be lighter than the image.
The red line is the small focusing image that is projected in the middle.
When you focus a small mirror is turned slightly so that the projected image moves within the finder.