What is the difference between a rangefinder camera and a SLR camera?

Rangefinder cameras are named "rangefinder" cameras because the viewing optical axis is located next to the optical axis of the photographic lens and parallel to each other.

Because of the parallax of the rangefinder in the picture above, it needs to be estimated before taking the picture.

The SLR in the picture below uses the 'pentaprism' principle to frame the view:

What you see with a SLR is what you get, there is no need to 'estimate'.

Of course, various cameras have their own advantages and disadvantages. For example, the wide-angle lens of a rangefinder camera can be made very close to the film, so the image is better than that of a SLR. The short, standard (50MM) focal length is its strength, while the telephoto is almost a dummy. The advantage of a SLR is its wide focal length coverage, and the rangefinder has unparalleled advantages in telephoto and macro. etc.