Exposure

Somtimes photographers use the term "Exposure" to mean the actual photograph, but strictly speaking, we are talking about the amount of light that is recorded by the film or sensor. Too little and you photo is under-exposed, too much and it's over-exposed.

There are several factors that affect exposure:

How bright the subject is;
How large your aperture is;
How long you leave the shutter open for;
How sensitive your sensor / film is (ISO).

Usually a photographer will vary any or all of these to get the desired effect, but often there are constraints - if you are shooting a landscape, the only way to control the brightness of the scene is to wait. If you are shooting film your ISO is fixed unless you actually change film or are carrying multiple cameras loaded with different film. If you are shooting fast-moving subjects you need a fast shutter speed to freeze the motion. If your subjects are not all the same distance from the camera you'll need a small aperture.

It's all a juggling act. Luckily, modern cameras automatically detect the brightness of the subject and can set the other values for you so you usually get a good exposure without too much effort.