I know a little bit because of my Leica infatuation with street photography but here are some things I found online.
A rangefinder camera is a camera fitted with a
rangefinder:a range-finding focusing mechanism allowing the photographer to measurethe subject distance and take photographs that are in sharp focus. Mostvarieties of rangefinder show two images of the same subject, one ofwhich moves when a calibrated wheel is turned; when the two imagescoincide and fuse into one, the distance can be read off the wheel.Older, non-coupled rangefinder cameras display the focusing distanceand require the photographer to transfer the value to the lens focusring
So basically I'd assume the camera's viewfinder looks like this while composing the image. Un-focused on the left, focused on the right
Basically from what I understand, there were rangefinder wars before just like there are dslr wars now with Leica's M series, Zeiss' Ikon, Nikon SP, and Canon 7. Then with the introduction of the SLR's rangefinders became a sort of niche market.
Rangefinders are really popular with street photographers such as Henri-Cartier Bresson because they look relatively unassuming compared to a dslr, and they allow the photographer to focus on the composition of the image more clearly (as some say). As far as I know they are quieter because they don't have a mirror slap that slr's do, adding to their discreet factor
Today I know Leica recently came out with the newest iteration of there M-series rangefinders, the M9. It's basically a FF rangefinder, and here it is mounted with a 0.95 noctilux
So basically you just look through that window on the left of the camera (even though on the canon 7 I hear the 0.95 is a bit in the way), compose your image (you can see what will be outside your image frame too, due to the rangefinder lines 28mm,35mm,50mm, etc) and fire.