The letters ISO on your digital SLR camera settings refer to the film speed. Even though your camera is most likely not film at all, but rather digital, the ISO camera setting still has the same function as older film cameras. Camera ISO determines how sensitive the image sensor is to light. If you're after a more technical explaination on 'What is ISO', Wikipedia has a good article called: ISO speed.
The most common ISO camera speed settings are: 100, 200, 400 and 800. Depending on your digital camera model you may also have them in the range of 64, 100, 160, 200, 400, 640, 800, 1600.
The lower the ISO number, the slower the speed. The higher the ISO number, for example 1600 the faster the speed.
General Rules and tips for ISO Camera Settings:
- Use an ISO of 100 or 200 when taking photographs outside in sunny conditions.
- If the sky is overcast or it is evening time, then use an ISO within the range of 400 to 800.
- Night time or in cases of low light you might need to set your digital camera ISO to 1600. If you don't then your photo will appear too dark if at all.
credit to : http://www.slrphotographyguide.com