Whether using a film or digital camera a photographer needs to understand how to manipulate ISO/ASA. The ISO/ASA number is a reflection of how sensitive the film or digital sensor is to light. The lower the number the less sensitive a medium, while the higher number reflects a greater sensitivity to light. This basic scale of ISOs is 100. 200, 400, 800, 1600, 3200. Each whole difference in setting is equal to twice or half the sensitivity difference depending on which way you are going. For example ISO 100 is the most common low setting which means it is not highly sensitive to light. ISO 1600 is a common high setting. ISO 1600 is sixteen times more sensitive to light than ISO 100. ISO 200 is half as sensitive as ISO 400.
This understanding is important because it allows you to chose the right ISO for any given situation. If you are at a dinner party you can set the ISO to a high number which will allow you to take images at a high shutter speed to avoid blurred images. If you are outside, you can use a less sensitive setting because there is plenty of light to capture an acceptable image.
Now choosing an ISO is pretty simple, but you need to know one more thing. The higher and ISO (i.e. the greater the number) the more noise in the picture. If your using film this is grain on the negative from large silver crystals. Noise is the same idea. It makes the image look blocked up through pixel distortion.
In this image I used an ISO rating of 200. Notice that despite the tight cop, you can see the detail pretty well and the shallow depth of field, especially at the top, make the cloth appear smooth.
In this image with the same aperture but an ISO of 1600, the sensor is 8x more sensitive to light. You can see the noise in the darker area and at the top, where the pixels look gritty as opposed to the smoothness in the other image.