Many cameras allow you to adjust the sensor's response to the available light. This setting, called ISO or Sensitivity depending on your camera, lets you get a brighter picture in dim conditions or a sharper picture when you aren't using flash.
Blurry indoor picture without flash at ISO 80
Sharp indoor picture without flash at ISO 1600
But there is a price to pay if you choose too high an ISO setting. You may notice colored speckles in your photo, especially in dark areas or sections without details such as a plain wall or sky. These speckles are visual "static," like the "snow" you see on your television screen when it is tuned to a channel not available in your area. You can see black and white dots on the TV but not the actual image. Noise in your digital photo is like that; it's visible but not actually picture information.
Indoor picture without flash with camera on tripod
The higher the ISO or sensitivity setting, the more likely your picture is to contain noise. Compact cameras generally produce more noise than SLR cameras at the same ISO settings. Compare these sets of images from the still life photo above. Both cameras are from the same manufacturer and both have the same number of megapixels. The images taken at ISO 100 look identical---no noise. But the compact camera photo shows more noise at ISO 1600 than the SLR camera image which is "cleaner" and "smoother" at the same setting.
Noise is hard to eliminate from your pictures after the fact. So use the lowest ISO or sensitivity setting possible to get the best quality photos. If your subject is stationary and you can place your camera on a tripod or other support, use the lowest ISO or sensitivity setting in dim conditions and you will still get a sharp, clear photo. If your subject is moving and a tripod isn't feasible (such as for photos of indoor sports), raise the ISO setting but realize that your picture will have some noise. Still, a sharp photo with some noise is preferable to a blurry one with no noise!
Blurry indoor picture without flash at ISO 80
Sharp indoor picture without flash at ISO 1600
Indoor picture without flash with camera on tripod
Noise is hard to eliminate from your pictures after the fact. So use the lowest ISO or sensitivity setting possible to get the best quality photos. If your subject is stationary and you can place your camera on a tripod or other support, use the lowest ISO or sensitivity setting in dim conditions and you will still get a sharp, clear photo. If your subject is moving and a tripod isn't feasible (such as for photos of indoor sports), raise the ISO setting but realize that your picture will have some noise. Still, a sharp photo with some noise is preferable to a blurry one with no noise!