What is ISO ?

It's the sensitivity setting for the sensor in your camera.

Rated by ISO:

• ISO 100 = less sensitive to light.
• ISO 800 = more sensitive to light.

Related: Aperture, Shutter Speed.

• Low ISO 100 = requires more light, larger aperture, lower shutter speed, less noise in photo.
• High ISO 800 = requires less light, smaller aperture, higher shutter speed, more noise in photo.
  
  

  Noise / Grain Effect	dpstudent.blogspot.com
  Low ISO 100, less noise, less grain.	High ISO 800, more noise, more grain.

In short:

• Go for lowest ISO possible, unless you want the grain effect.

 

References:

• http://www.photoxels.com/tutorial_iso.html
• http://www.dpreview.com/learn/?/key=sensitivity
• http://dpstudent.blogspot.com/2009/05/what-is-iso-setting-on-my-digital.html