I'm in the mood to type, but you can check this thread for some really great 411
http://www.thephotoforum.com/forum/...tering-white-balance-jpeg-vs-raw-formats.html
Exposure is a relationship b/n ISO, shutter speed (SS) and Aperture
White Balance is color.
If your image is overexposes, it might be perceived as change in color but it is really overexposed; opposite is true for underexposure.
ISO - sensitivity of the sensor to light. The lower the # the less the sensitivity, the better the less noise; opposite also in effect.
SS - the slower the more motion blur but more light enters
Aperture - the wider - the more light enters BUT the less dof you get.
The lens you're describing is probably a lovely cheapo kit lens 18-55 3.5-5.6 where at 18mm it's open at 3.5 while zoomed at 55, it closes down (stop down) to 5.6; again, color doesn't change (shoot in raw --> underexpose during shot by 2 stops --> correct in PP and you'll see that it doesn't change).
Now you mentioned about prime lenses and low lighting. If you are to do a search, probably just for the month of December 2009 till now, this Q came up time and time again and answer is really subjective.
Personally, after working with medium format, as great as the IQ is, one thing that I hated about it is caring 5-6 lenses on the job. Even at home, shooting s family portrait vs friends hanging out meant at least 2 different focal lengths. So once again, for me,
primes are OUT of the question.
However, a trend I noticed about me is that every 3-4 yrs I change my style of shooting and I'm coming up on 3rd change so we'll see what happens

Thus, I believe that your next purchase should be a flash unit. You can have the widest lens possible but w/o any light, your image will be dark unless you'll leave shutter open for an hour (j/k, lol, i hope you're getting an idea where I'm going with this)
My shooting style, I shoot at f/8 +/- a stop so I don't have a
NEED for lens wider then 5.6. That is why I am a proponent of 18-200; HOWEVER, you have to boost the sharpness in-camera b/n by default it is a very soft lens.
So, the final result will be what
you want to get out of photography and what your style is.
Good Luck