The Leica M8.2 is a rangefinder, first and foremost. This term is thrown around loosely to mean any camera that is not an SLR, but that is really sloppy terminology. A rangefinder is a very specific type of camera that allows you to frame and focus accurately, without looking through the same lens that the film or sensor is “looking” through. This, of course, has pros and cons, but the single greatest advantage it offers is the ability to see the subject without the view being affected by the aperture of the lens.

When viewing through an SLR, the lens is automatically set to the largest, wide-open aperture so the viewfinder is bright and visible. If looking through the lens at f/16, for example, the image would be too dark to see, and focus would be all but impossible. The lens, at wide-open, shows the image with the shallowest depth-of-field. This is great to help achieve a critical focus, but not so good for shooting at a small aperture, like f/22, when you want to see what your large depth of field will look like. Looking through a rangefinder, you can see how the subject will look without the shallow depth of field being imposed by the taking lens. You can also see to focus with a rangefinder in much lower light conditions because you are not stuck looking through the taking lens.

Far from being a curiosity, this ability to visualize and shoot based on hyperfocal distance (the maximum possible depth of field resulting from your focus setting and your aperture setting) creates a significant and very influential shooting style, and it comes from the Leica rangefinder legacy. Many photographers use this style. One of the more well known is Costa Manos, of Magnum.

LCD & Viewfinder: 
The Leica viewfinder combines a simple window showing the subject frame and cropping lines that indicate the view of individual lenses—ranging from 24mm to 90mm—with the rangefinder window in the middle of the frame. Using the lever control on the camera, you can switch frames without switching lenses, thus revealing a preview of what another focal length lens will give you. This is from the Leica manual sand shows the larger 28mm lens frame and the smaller 90mm telephoto frame inside:
 

The focus window gives you two overlapping, transparent images and allows you to line them up by turning the focus ring.   

Canon 5D Mk II	Leica M8.2

Canon 5D Mk II	Leica M8.2