Exposure

Exposure in digital cameras has three components to potentially change the exposure. These three components are aperture size, shutter speed, and ISO.
First it is good to understand aperture size. The aperture is simply how big the opening is for light to hit the sensor and collect an image. If all other settings are constant and the aperture is the only component being changed, the lower the aperture the bigger the opening and thus more light. If this blows parts of the image out you are able to raise the aperture number thus making the opening smaller and letting less light through to the sensors. Just remember the bigger the f-stop (what aperture is measured in) the smaller the hole. It is also important to keep in mind that aperture changes depth of field. This is why we have three components for exposure so we can find the right balance.
Next is the shutter speed. The shutter speed is the timing of how long light will be allowed through the size of hole (or aperture/f-stop) that you chose. Once you think about it, it is easy to figure out the longer the shutter is open the more light that will hit the sensors. And the opposite, the shorter the shutter speed the less light that will be allowed through the aperture.
Finally we have ISO. This is basically the sensitivity of the pixels. Most photographers stay in the lowest ISO settings as much as possible because it has less noise. Yet the higher ISO settings are good for night shots and picking up the small details. You just have to remember that the higher the ISO the more noise will enter the shot.
It is best to see if you can get the shot by fluctuating the aperture and shutter speed before you choose to lower your ISO. You'll be much happier with your shots and how sharp they are. The best way to get these things in your head is to take your camera out and practice. Sit in one spot and take the shot with changing all of the settings. You'll figure it out pretty quick :)