Yes, those questions are already answered many times in many books (I liked the National Geographic ones but there are cohorts of others, let's only cite Ansel Adams
The Camera), but still, I find it quite interesting for me to try to answer them at least a bit clearly.
The parts in italic are second-order effects and may not be as evidently visible as the others. I won't post illustrations - for that, I'd hope you won't mind me linking to
this page - it may first look like a dumb satire, but not quite, and indeed well-carved odd-degree humour, meaning there is some part of truth on either sides of the rules (as others have pointed, some left-side images are gems).
• Aperture can be raised or lowered to the limit of the lens. Why would one raise it? Lower it? Get a lens with a higher maximum or a lower minimum?
First, get that the lower the number, the bigger the aperture (you can verify that with your depth-of-field-preview button or an old manual lens).
A bigger aperture (smaller number) :
- lets more light in (useful when light is scarce), and
- makes a narrower depth of field (double-edged sword : can isolate a subject from a distracting background, but can ruin a picture if the
background tells some part of the story and asks for a more precise focus).
An aperture at the widest (smallest number) generally makes for poorer sharpness, and at the narrower settings (number above say f/16 or f/22) also makes some small blur and loss of contrast due to diffraction, so the best for sharpness is to stay around f/8. That said, sharpness is not always a major factor in an image's strength (think of the iconics photos of Robert Capa : they're f- unsharp).
• ISO can be raised or lowered within a range determined by the camera body. Why would one raise it? Lower it? Get a camera body with a higher maximum or lower minimum?
For the ISO, aim at the lower that the other parameters allow, to avoid noise. Make tests with your camera to see for yourself at which ISO noise begins to be annoying.
• Shutter speed can be raised or lowered within the range determined by the camera body. Why would one raise it? Lower it? Get a camera body with a higher maximum or lower minimum?
If anything moves in your image, you'll have to use a faster speed to freeze that motion, otherwise it will appear as a blur on the image (and that blur needs some fair amount of control to look good).
The same applies for the motion of the camera in your hands - for that, it is generally advised to set the speed at the inverse of the focal length. Id est, with a 50mm lens, don't go slower than 1/50s
(and 1/100s is generally still sharper with a DSLR, particularly with smaller sensor like Nikon D90 or Canon Rebel).
• What changes to any of the above effects the others? How?
The mix between these 3 variables determine the amount of light reaching the sensor, and the exposure of your image. Too much and it's white, too little and it's black, in between it may look right -
the best is to aim for "nothing plain white, but almost", as explained here.