We've all been in the same situation. We are attending an event where the lighting is poor but the moment is great and you absolutely have to get the pictures. Flash is prohibited and the first 16 shots you've taken with your digital camera are dark and look awful. Now what? Change your ISO!
ISO references how sensitive the camera is to using the available light to produce an image. For example, if your camera is set at an ISO of 100 then that is the equivalent of 100 streams gathering light through the lens to make the image on the film or digital card. If the camera is set at an ISO of 200, now you have 200 streams gathering light. As the ISO number is increased, you need less light to make the image because you have more streams to gather the light. This is why ISO 50 or 100 is typically used in day light situations where there is a lot of light. ISOs of 200, 400 or greater are used indoors or at night when the lighting is poor.
As with everything, there is a downside to all these streams gathering light. So before you jack up the ISO to 1600 and leave it there, you need to understand that the more streams you are using are truly making the camera more sensitive. The increased sensitivity causes unwanted noise' in your photos. Noise shows up in photos by making the pictures look grainy or just not crisp. It is a trade off. If you really need to get the pictures, bump the ISO one setting at a time and test the image to see how grainy it looks. It is possible to find a good balance of lighting and noise. If your photos do end up with a lot of noise, not all is lost. There are software programs that can do a pretty decent job at cleaning up the photos. They work with varying results but are worth trying if there are moments that you want to make sure are preserved noise free.
Altering the ISO is mainly used in manipulating low light situations but it can also be used when you want to stop the action in a photo as well. Consider again the number of streams letting light into the camera at 100 vs. the number of streams at 200. If you have twice as many streams capturing the light, this means that your shutter speed will be quicker. (The shutter is the mechanism that actually lets light into the camera.) Fast shutter speeds are the key component in freezing action in pictures. Therefore, if you want to get that clear picture of your 6-year-old running towards the soccer goal net, one way is to increase the ISO setting. You still need to find that balance between noise and the picture you want but it is another option available to you.
ISO is only one piece of the puzzle in capturing great photos. However, in low light or indoor environments where flash cannot be used, it is often the key piece. Increasing ISO may be the difference between getting the shocked look of Grandma's face when she walks into a surprise party for her and getting dark red-eyed blobs that look like something out of a scary movie.