Rangefinders are generally cameras that use a separate Rangefinder for focussing connected to the viewfinder instead of direct focussing thru the lens or an additional focussing lens (as with a TLR). The focussing through a rangefinder is different as the image on the whole viewfinder would be in focus the whole time... focussing is done by aligning two images right smack in the middle of the viewfinder together, forming a clear, proprely aligned image in the center. (Might be hard to understand, but you really need to use a rangefinder to actually know what i'm talking about).
Leicas are the epitomy of premium rangefinders, used by many generations of photographers in the past as well as in modern times.