ISO Sensitivity regulates how much light gets into the camera. The higher the number the more light will enter. This picture was taken in very dark room at 1600 ISO. Now along with a brighter pictures comes negatives. The picture will appear to be grainer and will take a long time to capture.
This picture is the same, just taken at a different sensitivity. I took this photo at the lowest sensitivity I could, which was 200. Any lower and the photo will not have been taken. Because the sensitivity was so low it took longer to capture and it was more sensitive to movement which explains the blurriness. Other than that the only difference is the amount of light.