ISO adjusts the sensitivity of your camera to light.  As in all photography there are compromises.  As you increase the ISO to make your camera more sensitive, there is a slight loss in quality (noise).  The higher you go the more you loose.

But modern cameras can work at high ISO values not imaginable only a few years ago.  Modern cameras can create very clean images at even 1600 ISO and higher.

I would rather have a image that has slight noise but is sharp, rather than an image that had camera shake, just because I didn’t want to put the ISO up and loose some quality.

Noise in an image can usually be dealt with in Postproduction, a unsharp image is much more difficult to save.

Next up Aperature….