ISO is the rating system for film or digital sensors in regard to light sensitivity. The technical side of all this is pretty detailed and lengthy (and, for the most part, unimportant), so you can read about it here if you want.
With film, the higher the ISO the larger the grains of silver are on the film (for the most part, anyway). High ISO film is "grainy" while low ISO film is not. Higher ISO films are more sensitive to light while low ISO films are less sensitive to light.
With digital, instead of "grain" you have "noise". High digital ISO is "noisy" while low digital ISO is not. Digital ISO interprets the sensor data in such a way that it has a lightness similar to film of the same ISO.
Common ISO choices are 50, 100, 200, 400, 800 and 1600. There are (or, in some cases, have been) other choices, too. Each change in ISO doubles or halves the light sensitivity. For example, ISO 100 is half as sensitive to light as ISO 200 and ISO 800 is twice as sensitive to light as ISO 400.
The reasons why one might choose one ISO over another depends on if he or she wants grain (or noise) or not. Most photographers prefer to have the finest grain practical and choose the lowest ISO they can get away with for whatever lighting situation they may be in. Some photographers, however, love grain and purposely choose high ISO for the effect.
Grain can look good when used carefully. It is more contrasty (there are fewer middle tones in high ISO film) and the texture can help add emotion to the image. Grain looks better in black and white than color, in my opinion. I also think that digital noise is slightly less attractive than silver grain.
I prefer low ISO and minimal grain or noise. I rarely photograph using a higher ISO than 800 (and even prefer not to use ISO 800 unless I really need to). I did, however, take a few photographs around the house with high ISO for this blog post. From the small images below it's difficult to see the digital noise, but if you click on them you can see a higher resolution copy where the noise is more obvious.
Shutter 1/25, f8, ISO 6400, Pentax K-x, 55mm
Shutter 1/8 (handheld), f8, ISO 6400, Pentax K-x, 55mm
Shutter 1/125, f9, ISO 6400, Pentax K-x, 55mm
Shutter 1/1000, f22, ISO 6400, Pentax K-x, 42mm
The lowest ISO available on my Pentax K-x is 100; however, on this camera, the difference in noise between ISO 100 and ISO 200 is very, very minimal. It's not until you enlarge to 16" x 20" (or 16" x 24") that you can even begin to notice, and even then it's not without close study. I don't worry about that small amount of noise. So I commonly use ISO 200 instead of ISO 100, which gains me some light sensitivity (one f-stop).
On the K-x, ISO 400 has more noise than ISO 200, but it's quite difficult to tell the difference unless the image is enlarged to a size bigger than 8" x 10" (or 8" x 12")--again, not without close study. ISO 800 has significantly more noise than ISO 200, but you can't tell the difference until enlarged to a size bigger than 5" x 7" and not without a close study.
Here are some examples of low ISO. Again, you can click on them for a higher resolution copy.
|Cronin House Peaks - Phoenix, Arizona|
Shutter 1/30, f11, ISO 200, Pentax K-x, 55mm
|Historic Cronin House, Built 1893 - Phoenix, Arizona|
Shutter 1/20, f16, ISO 200, Pentax K-x, 30mm
|Top of Cronin House - Phoenix, Arizona|
Shutter 1/25, f16, ISO 200, Pentax K-x, 55mm
Your ISO choice will depend on the lighting situation (do you need more light sensitivity or not?), the effect you desire (do you want minimal grain or noise, or lots of grain or noise?), and what you intend to do with the image (how big will the image be enlarged?). If you don't want grain or noise, choose the lowest ISO that will still give you the light senstivity that you need for whatever you are photographing. If you want grain or noise, choose a high ISO that will give you the effect you want. If the image won't be enlarged more than 5" x 7", it will not matter much which ISO you choose and you might consider shutter speed or aperture choices more important--the smaller the photograph will be, the less important the ISO choice is.