ISO : photographic speed for negative film but since digital camera dun use film,
they put the equivalent rating for it (eg: ISO 800 equivalent)

eg:
ISO 100: more shutter time (less noise) -> stationary object, normal lighting cond.
ISO 800: less shutter time (more noise) -> good for moving object or less light conditon

i never use ISO 800 and above but i think it's oredi enough for normal picture taking..