Well, if ISO determines how sensitive your sensor is to light, and your sensor records the light (a fancy way to say records the finished image) then that sensitivity of the sensor is basically going to determine (along with your aperture which determines how MUCH light hits the sensor and shutter speed: which determines the DURATION OF TIME the light is exposed to the sensor) how bright your image will be (how much light will be absorbed by the sensor).
Film is composed of lots of little circles that make up the GRAIN of the film. Digital photography is similar, except for that our little circles aren’t circles at all, they’re squares. . . a gazillion little squares that come together to create an image. Sometimes those little squares in the image become slightly visible and this is referred to as NOISE. Check it out in the image of my son above.
