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One of the reasons why I use a high-end digital SLR (currently the Canon 5D Mark II and Canon 7D) is to get the cleanest possible image, that is, an image with as little noise as possible. I like to keep it clean, so to speak.
In my quest for a clean image, I always shoot Raw files, and use the lowest possible ISO.
In addition, I strive for the best in-camera exposure, trying not to underexpose the image, which can add noise to an image, especially in the shadow areas.
At its worst, underexposing can cause what is called "banding" in a file – an effect in which you can actually see the bands of pixels. Banding, by the way, is exaggerated with JPEG files, which is another reason for shooting Raw files. (But as someone just pointed out (and reminded me) on twitter, you can get banding even with a Raw file if it's poorly exposed and processed.)
I actually don't have a good example of banding because, again, I strive for the best in-camera exposure - always checking the histogram on my camera's LCD monitor. So, in an effort to illustrate banding (and noise), I opened up the shadow areas from one of by Botswana images using Curves. The long white boxes illustrate the most visible bands in the picture, which you probably can't see because it's a low res file.
I used this image to illustrate this point: If you underexpose a file too much and try to open up the shadows, you'll get an image with noise and banding.
About my properly exposed elephant image: check out the tonal range. The sun in not washed out and you can see into some of the shadow areas. This is an example of why I say, "Raw Rules!"
Speaking of noise, if you do have a noisy image, Topaz DeNoise does a great job of reducing it.
On another topic, check out the ghost image of the sun in the top picture. It was actually in the bottom picture, too, but I removed it with the Burn Tool and Clone Stamp tool in Photoshop.
The ghost image was caused by the sun reflecting off the front element of my lens and then bouncing back onto my UV filter. This is when I leaned that you gotta remove all filters when shooting into the sun :-)
Explore the light,
Rick