I get a lot of questions about ISO--many photographers don't seem to understand exactly what it does. Your camera's ISO control determines how sensitive the camera's sensor is to light. On most cameras, ISO starts at 100 and goes up from there; the higher the number, the more sensitive the sensor will be.
Of course, that begs the question: Why wouldn't you always just leave the ISO as high as it can go all the time?
That's because ISO is a bit of a mixed bag. Higher ISO values give your camera a better light response, so you can take sharper photos with shorter shutter speeds in low light, but this comes at the expense of more digital noise in your photo. On the same camera, a picture captured at ISO 800 will tend to look noisier--random pixels that resemble grain on an old film camera or static on a television screen--than a photo shot at ISO 100. is an enlarged detail of a photo taken at ISO 1000. Notice the rough, sandpaper-like quality of everything in the scene, including the wall and the girl's complexion.