When you raise ISO from 100 to 200, you double the output of the CCD sensor.
Raise it from 200 to 400 you double it again.
It is much like raising the volume on a small portable radio.
The higher you raise the volume, the more distortion you get.
Turn that small radio all the way up and the distortion is overwhelming.
ISO speed, shutter speed and aperture all work together to get the correct EXPOSURE.
So you need to understand Exposure.
Here is a link:
After you understand that, then read about ISO:
After reading the title, you may be thinking, "Ooh boy, we got ourselves a newbie here!" Don't worry, it's not that bad.
I know what it does. I know what it stands for ("International Standards Organization"). I know that ISO is a measurement of light sensitivity. I know that upping the ISO setting is very useful in low-light situations (to create "light", or where the subject is moving very quickly (to reduce blur). I know that various other factors affect the effectiveness. I know, also, that factors such as light-gathering ability (e.g., bigger pixels/photodiodes) affects the degradation that results from too high an ISO setting. I know that ISO is a give-and-take, where the higher the setting, the more "noise" is introduced into the image.
WONDERFUL!!! But how does it all work? Why does a higher ISO result in more noise? How does a higher light sensitivity reduce blur? etc., etc.