It amazes me how many people are completely oblivious to the ISO feature of their SLR. When they do find it, the common reaction is "Hey! If I keep my ISO at 1600 I can always shoot with a wicked fast shutter speed! WOW!"

But, eh, not really. You see, the ISO function determines the light sensitivity of your sensor. So, in overly bright settings, the ISO can be set to as low as 100 since there is no need to become even more light sensitive. Darker environments are where a higher ISO is better suited, because your camera will detect more light this way.

When you need a slow or fast shutter speed, and the light conditions simply aren't proving for it, you may have to change your ISO setting. When there is more light, a faster shutter speed is ideal, and vis versa in low light conditions. However, if your images are overexposed with a slow shutter speed on a bright day, you may want to lower your ISO level. When you want a faster shutter speed on a gloomier day, you may have to increase your ISO. We all know low-light shooting can be a pain in the ass, and this can reduce the risk of blur.

Unless you want grain, which may occasionally be the case, shooting at a high ISO isn't usually preferable. The higher the ISO number, the more noise/grain you will see in the image. When you need to enlarge an image, which can cause pixelation, grain doesn't help.

My personal technique? Shoot at the lowest ISO that works for what you're shooting. For general use, 200-400 is usually ideal.

Have fun shooting - experiment a little!