>>4105199noise is literally noise. correctly called read noise. it's unwanted electrical charges that aren't related to the signal the sensor should be picking up. you get more noise with a higher ISO because ISO on digital is just decibels of signal gain that correspond roughly to the effect of moving up to that sensitivity with film. more gain and you amplify the noise, making it more noticeable.
grain is literally grains. B&W film photographs are made out of millions of tiny grains of silver, developed from light-activated silver halide crystals. color films have chunks of dye instead that are converted from light-activated silver crystals paired with dyes. film with smaller grain can record more detail but takes more exposure to light to activate enough of the tiny crystals (grains) since each grain receives less light. more time in a developer bath will give less-exposed crystals more time to be converted into solid grains or silver or develop into fixable dye clouds.
