Lynette:
Just to add a few points ...
Rangefinders can be divided roughly into two types - those that have interchangeable lenses, and those that have fixed lenses. I would guess that Rol Lei Nut's comments were intended to mainly apply to cameras like Leicas (which you asked about) which allow you to change lenses, but there are some things to be aware of when you consider fixed lens rangefinders instead.
With all rangefinders, you don't need a mirror and prism to allow you to view through the taking lens. Instead, you view through a viewfinder which is separate but close to the taking lens. Usually, the viewfinder includes a focus aid (a rangefinder - thus the name). As the viewfinder is separate from the taking lens, it can be made brighter and larger.
As a mirror and prism isn't required, lenses can be closer to the film. This gives lens designers certain options that can result in smaller and lighter and simpler lenses, without loss of quality.
When it comes to fixed lens rangefinders, there are a couple of other advantages. Many fixed lens rangefinders have the shutter in the lens (a leaf shutter). This permits very quiet operation, and use of electronic flash at all shutter speeds.
In addition, a fixed lens rangefinder doesn't have to include complexities like variable frame lines in the viewing system.
If you are unsure about what it is like to work with a rangefinder, it is a very good idea to try out one of the simpler fixed lens rangefinders. They were very common in the 1960s through the early 1980s, and can be purchased used (or even found for free). My most recent acquisition is a Canonette with a 1.7 lens which I found at a garage sale for $10.00. There are many comparables, and lots of threads on APUG that discuss them.
If you like working with something like a Canonette, you may love working with a Leica.
Have fun!