An SLR camera allows you to look through the lens and was created to reach WYSIWYG (What You See Is What You Get). It has a mirror box inside, and as a result, is much larger. Other developments include splitting the beam for auto-focus, etc.

A rangefinder is a camera that has a rangefinder mechanism. This is a device that measures subject distance. Through this device, you see two images. When the two images coincide through moving the dial, the correct distance is displayed. On older cameras, this was a separate device and one must transfer this to the lens. Now they are built into the viewfinder. You have different viewfinders for different focal lengths (zoom lenses are difficult, as a result)

Advantages:

• body size/weight
• discreetness
• no mirror blackout, mirror sound, mirror induced vibrations
• shorter registration distance: smaller/lighter lenses, potentially higher quality wide angle lenses
• ease of both-eye-open photography and awareness

Disadvantages:

• lack of autofocus (though some have contrast-detect AF, but it is not phase-detect)
• parallax effect, pronounced at close distances
• no depth-of-field preview, exact framing, and other WYSIWYG things
• switching viewfinders