The aperture is the small round hole in front of the camera lens. Its primary function is to control the amount of light that enters the camera. Many digital cameras use an aperture that is variable. Therefore, use a larger aperture, or a larger aperture range, as in the case of some digital cameras, for low-light situations. The shutter speed is another way to vary your exposure.
Shutter speed is simply the duration of time that the shutter stays open.
Thus, the longer the shutter stays open at a given aperture, the more light that strikes the subject, and the less chance your picture will be underexposed. A third method to increase or decrease exposure is to adjust the ISO or ASA speed. The higher this rating, the more light sensitivity the camera will have. Now you're on your way to a perfectly-exposed shot. While your camera's autoexposure setting will probably do well in properly exposing the majority of your shots, here are some tips for real life instances that can give you an edge in taking better photos.