ISO is just the sensitivity of the sensor in your DSLR, in the film days, it is the sensitivity of the film, due to different types of chemicals being used to achieve different sensitivity towards light.

Usually you don't need to set ISO most of the time, usually normal sunny daylight just set it to ISO 100 or 200.

If you want to "freeze" high speed action, then set to higher ISO like ISO 400/800, then couple with a higher shutter speed, like 1/1000s above

If you're in darker room and don't want to use flash, then using higher ISO like 1000 or 1600 will make your pic brighter but at the cost of making your pic look very grainy.

Usually you just let the ISO settings to be constant most of the time, you will only have to play around with either Aperture/Shutter Speed or combination of both to get the best results.

This post has been edited by geekster129: Sep 22 2010, 11:50 AM

lol.. Big aperture is a double-edged sword. Good bokeh, but if mishandled, shallow depth-of-field is good enough to ruin your pictures, if you're intended to take group pics, which is far away from your camera.