Well, somebody correct me if I'm wrong, but I'm pretty sure the main difference between an SLR and a rangefinder is in the way the photographer looks through the lens to frame the shot. In an SLR, when you look through the viewfinder, you are actually looking directly though the same lens that focuses the light onto the film. In a rangefinder, when you look through the viewfinder, you are looking though a totally different (and smaller) lens. Because of this, rangefinders can have what's called parallax error, where the framing of the subject in the viewfinder is slightly different from what hits the film, since the viewfinder and main lens are in different locations. However, rangefinders are generally cheaper and smaller than SLRs because they require less mirrors and moving parts to get the image to the viewfinder. In general, with 35mm cameras, typical consumer point-and-shoot cameras are rangefinders, while more advanced cameras tend to be SLRs.
I hope I'm not way off on this, but I'm pretty sure that's what the difference is.