not 100% sure how it works with the digital cameras but ISO is about film sensitivity and film speed, basically by changing the ISO youd let more light in or let less light in, its kinda how by changing the shudder speed you can do long exposures and stuff like that for everyday pictures the automatic is usually the best unless you know what changing the ISO will do.
for example for the iso and shudder speed...
its kinda of how your eye works, when its dark your pupil gets bigger and when its bright it gets smaller. when the iso is set to automatic it will do that all on its own, if you were to change it you can make the "pupil" stay opened even when its bright, or make it smalll when it dark. the shudder speed is kinda like blinking, your shudder speed is how long your "eyes" would be open, before blinking. so the longer your shudder is open the longer the light will pass though, the less its open the shorter the time the light will go through.
that help?
