1,I know that lowering your ISO makes your picture darker, but does it also improve the image quality?
Well that's not exactly true, unless you are set to Manual mode with a fixed shutter and aperture setting - then you will notice the photo getting darker as ISO goes down, or brighter as it goes up. The reality is that ISO is one of the three primary tools used to accomplish exposure, along with shutter and aperture. Lowering the ISO reduces the 'gain' of the sensor, meaning in order to get more brightness, you need to get more light to the sensor - by either using a slower shutter speed or a bigger aperture. Like anything when the 'gain' is turned up, things get noisier or grainier (ever deal with old HAM, CB, or shortwave radios and turn up the 'squelch' or gain knob? - more static noise but more sensitivity to fainter signals. ISO works the same way). In general, most photographers agree that the best approach to ISO is to use the lowest ISO you can to still get the shot you need. Manipulate shutter speed to be as fast as you need it to be, or aperture as open or closed as you need for depth of field...only if those two can't be optimally set do you then go to ISO to boost it up until the other two parameters are where you want them. The short answer to part 2 of your first question is...yes - lower ISO delivers much better quality, with greater detail, color fideltiy, and lower noise.
2, I normally take pictures that come out as JPEG pics (large 18MP 5184x3465), but what about RAW pictures? When are you supposed to take pictures using RAW + L? and why?
This always opens up a floodgate in camera forums. The problem is that there tend to be three very distinct groups of thought:
1. RAW is the ONLY way to shoot, you are not a professional unless you do, you are only 1/2 a photographer, you're a bit daft too, clearly ignorant, blind, and a bit stupid, and will never realize how poor of a photographer you are until you learn to shoot and process RAW.
2. JPG is the ONLY way to shoot, RAW is a waste of space, waste of time, only for lame photographers who can't get it right in camera so they have to make up for it with processing, RAW supporters are egotistical judgmental fools, techheads who prefer time spent in front of a computer creating their photos than out taking them.
3. (and this group is very small, usually lost in the argument of the above two groups) - Both JPG and RAW are optional ways of shooting, both with their own positives and negatives, and each person needs to decide for themselves what format is best for their needs, their time, their photography, and their results. Also, very fine photographers can get excellent results from both formats, a photographer who gets things right in camera and knows how to set up their camera will do better in either format than someone who doesn't, and accepting that JPG has advantages in convenience and space and RAW has advantages in editing latitude and processing flexibility.
It's more than likely that you are going to hear from one of the first two groups who is going to try to convince you that despite all of this, you really should be shooting in * (RAW/JPG) because you * (are better off learning how to control and shoot with the camera and not relying on the computer to fix your problems/will never be a proper or professional photographer or realize your potential if you let the camera make the image for you).
Sony DSLR-A580 / Sony 18-250mm / Minolta 50mm F1.7 / Sigma 30mm F1.4 / Tamron 10-24mm / Tamron 200-500mm / Tamron 90mm F2.8 macro / Minolta 300mm F4 APO
Sony NEX5N / 18-55mm F3.5-5.6 / 55-210mm F4-6.3 / Pentax K adapter / Konica K/AR adapter / bunches o' Konica & Pentax lenses!