Exposure

Digital SLR Cameras

Lower shutter speed = more light

Higher shutter speed = less light

Low aperture = smaller "in focus" range and more light

High aperture = larger "in focus" range and less light

When using the manual setting, you need to balance the two to come up with the correct exposure. If you can't get the exposure you want by doing that, you resort to the ISO.

Higher ISO = more light, but consequently more noise

Lower ISO = less light, but get clear and crisp images

If using the Aperture Priority or Shutter Priority settings, then set one of the settings to whatever you're needing and the camera calculates the other for you.

If you can't get the exposure you want on your camera, editing tools work quite nicely.