I'll limit this to only the concept in a digital world. Treat ISO as a sensor sensitivity setting (say that fast five times). The higher the number, the more sensitive the sensor is to light. The tradeoff is that, as you crank up the ISO, you'll intorduce more "noise" into the image. Some people will call this grain but it's not the same thing. Suffice to say more noise will look like larger artifacts and dots in your exposures. Take a long exposure of the night sky with no processing on it and you'll see a blue/black speckled frame in most cases, that's noise. You use lower ISO in brighter scenes normally while the higher ISO settings are saved for night or inside pictures. You might use a higher ISO setting outside to use higher shutter speeds to stop action too. Outside daytime in the parks I would say 100, inside, maybe 800 depending on available light. 1600 to 3200 for dark rides and no flash. I know I said higher ISO can be noisy but sometimes that's the only way to get some pics. Note that night/very low light exposures will take TWICE the time of your shutter speed. Say you take a 5 second exposure - your camera will take another 5 seconds to process (it's doing something called dark frame subtraction, trying to optimize the image with onboard s/w). Some cameras are better at that than others. Some cameras have better sensors than others. ISO on your camera will not be the same as on others (close but not exactly the same).
HTH
