A rangefinder camera is a camera fitted with a rangefinder: a range-finding focusing mechanism allowing the photographer to measure the subject distance and take photographs that are in sharp focus.
Because there is no flipping mirror, lenses can be designed without having to keep the back of the lens far enough away from the image plane to avoid getting hit by the mirror allowing for a smaller lighter camera.
For wide-angle lenses, allowing lens designers to make lenses whose rear elements can come very close to the image plane lets wide-angle rangefinder lenses be much smaller, sharper and have less distortion than SLR lenses.
Because there is no flipping mirror, there is much less vibration to blur hand-held images. Rangefinders do without the extra weight of prisms, focus screens and flipping reflex mirrors.