Exposure is a balancing act of three elements, shutter speed, ISO, and f stop. Quite often one of these elements will be chosen, at the expense of another.
Shutter speed is how fast the lens opens and closes and therefor, the slower the shutter speed the more light hits the sensor. A fast shutter speed allows less light into the sensor.
In digital photography ISO measures the sensitivity of the camera sensor. The lower the number, the less sensitive your camera is to light and there will be less grain. At low ISO i.e. 50 or 100, your images will have little grain and beautiful crisp shots, with strong colour saturation. Higher ISO settings are used in darker situations, so that a faster shutter speed can be used, however, higher ISO settings give more grain.