One of the "buzz" words in digital cameras today is high and higher ISO capability.
While high ISO numbers allow better low light capabilities an increase in "noise" is often the result.
ISO figures can be very easily compared to ASA numbers from the days of 35mm film.
Shoot 400 ASA film and you will have more "grain" in the final image when compare to ASA 100 film.
The "grain" in today's images shot at higher ISO setting is described as "noise".
For the most part, many compact digital cameras start to see a decrease in image quality at around 400 ISO. Many DSLR's start to show unacceptable levels of noise pollution at 800 to 1600 ISO.
A few of the higher end, full 35mm sensor sized cameras can handle higher ISO numbers well, but with ratings up to the 25,000 ISO mark, even these fantastic tools of photography can produce substandard image quality at the highest ISO settings.
One of the problems that is associated with the high ISO numbers is one of the processor in the camera attempting to reduce the noise in the image at the expense of image sharpness.
I look at ISO settings in my photography work much the same as I did ASA numbers when I was shooting film.
If I want the highest quality and sharpness from an image with the least amount of "grain", I use the lowest ISO setting possible.
If I want a little grain in the photo (B&W photos can sometimes benefit from this technique) then I can start to "crank up" the ISO.
If I need longer shutter speeds to make the shot, I grab a tripod long before I raise the ISO much past the 400 mark in any of my compact cameras and limit the Fuji s5 to around 800.