Rangefinders are a categorically different beast from the DSLR when it comes to focusing (and in some cases, metering). Rather than having a multitude of focus points and light confirmations on focus, you get a small patch in the center of the viewfinder. This takes some getting used to if you're coming off a DSLR world. Also, metering tends to be a big question for me lately, so I'll touch on that as well.
When you look through any modern DSLR's viewfinder, you'll see 1.4 billion focus points. OK, not that many. But modern DSLRs are priding themselves on the number of available autofocus points they give you. You can usually choose to use a single, moveable point, and now upwards of 51+ active points to pick from. This gives the photographer the freedom to compose the frame first and pick a focal point later. You use your dial and move the focus around (usually highlights the point of choice in red) to the point of your choice and half-press the shutter release for a confirmation light to tell you it's sharp. Click and move on. This is a great tool to really help speed things along and let the computer chip confirm you have the shot. It's not foolproof, but these systems are getting more and more intelligent all the time.
On the rangefinder, you get a small box (or circle) in the center of your viewfinder. Inside that box is a split image of the center of your frame. You adjust the focus until the center is sharp (according to your eye, nothing else). At times, this can be really challenging if what you want to focus on is not dead center of your frame. And really, how often is what you want to focus on dead center? In these cases, you must first focus your shot by centering the viewfinder on the desired 'sharp' subject and then recompose it to your likeness - making sure you don't sway forward or backward, taking it once again out of focus; especially a problem when shooting wide-open without a tripod.
If you look at the image above to the right, you can see the frame on the left shows that the center of the screen isn't lining up. This means you are not in focus. You rotate your barrel until the image appears sharp, then you can click away. Yes, the image to the right is a little hard to see - and it's just as hard through the rangefinder! But trust me, you get used to it.
Light metering is philosophically the same on a rangefinder and a DSLR. Modern rangefinders, like the Leica M (type 240) offer various metering modes like modern DSLRs: matrix, centered and spot metering. But the majority of today's rangefinders on the street only offer center weighted metering. For my older Leica M2, there is nothing on board for metering and I use either an old analog light meter or one like LightMeter (pictured right) on my phone.
Let's handle the rangefinders with built-in meters first.
Modern rangefinders like my Leica M-E / M9 have a built in light meter which is only center-weighted. For these cameras, the center 1/3 of the sensor acts as the meter, analyzing the amount of light coming in and indicates if you are under/over exposed for the shot. The issue is when you have both bright areas and shadow areas of light falling onto the sensor for a single image. The meter will take the average of all that light and indicate the exposure. This is not always desirable.
On a DSLR or a Leica M (type 240) you can switch to spot metering to evaluate only a small portion of the sensor and give you a more accurate exposure for your needs. So how do you accurately expose for just the shadows or just the highlights? Well, my trick is to actually point/move the camera to the area I want to expose.
For example, I wanted to expose for the shadows and damn the light coming in. When I framed the shot, the rangefinder was pulling in all that extra sunlight and was going to severely underexpose the rock wall. So before I shot this, I dipped the camera below the wall and metered the light there, then walked back with my settings locked in to compose the image. Result was a well exposed wall - even if the sky is slightly blown. (Zeiss Ikon / Portra 400 / exposed ISO 200).
This brings up a difference in metering that I touched on earlier: metering for film. This goes for all film no matter what film you are shooting (35mm, 120 and so on) and what type of camera you are shooting with (SLR, rangefinder, etc). And that's to meter to half the box speed.
Say you are shooting a Zeiss Ikon (where you can set the film ISO) and Portra 400 film. Simply set the ISO on the Ikon to 200 and meter like normal with the camera. If you are on something that will not allow you to set the ISO (Leica Mp/M2 or Hasselblad 500c for example) then set the ISO on your meter to 200; half the box speed rated at 400. The first time you do this you'll swear you just shot and ruined an entire roll of film. But when it comes back - you should be surprised and in love with the results.
Film can handle a much larger exposure range - especially over exposure - compared to digital. I'll typically go 2-4 stops over in exposure to gain warmer, more earthy tones and still have (in my eye) amazing results. If I did the same on my M-E, the highlights would be completely blown out. Come to think of it, I'll have to do a comparison for you later on - meaning shoot at the box speed, then half the box speed, then half the box speed minus one more stop.
Hopefully this quick article gives you a little insight into focus and metering on rangefinder bodies and how it tends to differ from D/SLR and other systems. This was by no means meant to be exhaustive or technical.