Low
ISO (50-100)
Use for bright days when you want the best possible quality.
Unless you're shooting with a very long lens you won't need
anything higher in fine weather. You can also use a low ISO
setting at night when you want a long exposure (such as when
capturing the blur of car headlight trails or shooting
fireworks). In such cases you'll need a tripod to avoid
camera shake.
Medium ISO (100-200)
Ideal outdoors on overcast days or when using a long lens in
bright weather. ISO 200 is often the highest setting you can
use without risking noise in your image.
High ISO (400)
For shooting in low light, or with a very long zoom or when trying to freeze fast action. ISO 400 is when noise first starts to become a problem, but it's usually preferable to have a bit of noise than the blur of camera shake.
Very High ISO (800+)
For shooting handheld at night, indoors without flash and in any situation where a lower ISO would cause blur through camera shake or subject motion. Noise is unavoidable at ISO 800 and higher (and most cameras don't even offer the option). That said, some of the very latest FinePix models (such as the F10) have class-leading high ISO performance, in many cases with lower noise than the ISO 400 on standard CCD cameras. Higher ISO settings can also be used to extend the range of your on-camera flash, and to allow more flattering portraits in low light (by turning off the flash entirely).