In addition to all those said... ISO 200 will require half the amount of light to achieve the same exposure compared to ISO 100... and ISO 400 will require half the amount of light to achieve the same exposure as compared to ISO 200.
So, when do you reach for that ISO button? Generally, use ISO100, this gives better image quality, less noise (Random nonsense pixels). Let's say you are indoors or the sun is setting... you pull out your camera, measure the exposure, and Oh god.... You need 1/10 seconds to achieve the correct exposure even though your aperture is set to the widest... and you don't want the flash to throw out the ambient light or for some other reason. And you didn't bother to bring that clumsy tripod or there's no space/time to set it up. So, reach for the ISO switch, switch it to ISO 200 and the shutter speed magically becomes 1/20 seconds, which may not be the best to hand hold but better than 1/10... But darn... due to the lack of breakfast... you are shaking like a drench cat... pump the ISO to 400 and you only need 1/40 seconds~ Nice~ Snap away~
With a higher ISO, you may not get the best picture, but you don't lose the picture(s)!