Are you shooting in full sun? 100 ISO Is it in the shade, but on a sunny day? 200 ISO Am I back in a darker shaded area outside? 400 ISO Am I in my home, but near a well lit window? 400 or 600 ISO Is it cloudy outside and in my home?
Photography Kick-Start Guide: Adjusting Your Camera’s Settings for the Photo You Want. Describes each camera setting in detail and what to use it for. Great for people looking to learn how to use their camera better.