Exposure

 Exposure is the amount of light allowed to fall on each area unit of a photograph, during the process of taking a photograph. Exposure is measured in lux seconds.
If the shot is exposed too long the photograph will be washed out. If the shot is exposed too short the photograph will appear too dark. 
These pictures I took to demonstrate the effects of over and under exposure as well as some in-between:

This is the most under exposed photo; as you can see it's far too dark, but illuminates the sky so can be good for silhouettes.

This is the middle photo in the spectrum (exposure 0), where the light is most balanced across the whole image; therefore making it the most commonly used when taking simple shots.

This is the most over exposed photo, where there is too much light being allowed to fall on the image; it's where the background tends to be too bright for what's in the foreground.