ISO is one of three determining factors of the exposure of a photo, along with aperture and shutter speed. These two affect the lens and exposure time respectively, with the ISO affecting the sensor (or film). To be more specific, the ISO determines how well exposed a photo will be by changing the sensitivity.
Here are some of the ISO settings, and an exposure triangle.
Here are some photos I took, showing different ISO settings, with Fast Shutter Speed and Slow Shutter Speed. (7 photos with fast shutterspeed and 7 with the slow one)
Here are 7 photos with the fast shutter speed, every photo has a different ISO setting.
Here are 7 photos with the fast shutter speed, every photo has a different ISO setting.