ISO refers to film speed (100, 200, 400, 800, 1600)—even if you’re usually a digital camera.
The lower end of the range (100 and 200) is consider slow, and is ideal for both still and nature shots. It gives you a less grainy picture than at the higher speeds and is better suited for enlargements. Since it requires lower shutter speeds (see tip #8), you might need to use a tripod. You also need to be cautious of windy conditions if you don’t want blurred pictures.
The higher range (800 and 1600) is great for action shots and less than ideal lighting conditions. It allows for a faster shutter speed. However, photos tend to be grainer, which can be a disadvantage when enlarged. It can also be a creative advantage, depending on your goal.