A digicam uses built-in light meters to measure the light reflecting off the subject. There are different ways in which the camera's metering mechanism calculates exposure. The metering mode you choose will depend on the particular shot you're taking.
This exposure system works by dividing the frame into a grid or matrix. Then it analyzes light at different points on the grid and chooses an exposure that best captures both the dark and light sections of the scene.
This method measures light throughout the scene but gives greater importance (weight) to the center of the image area, assuming that that is where the primary subject is located.
This arrangement measures light throughout the scene but gives greater importance to the bottom of the image area.
This gives you the most control and almost allows the camera to operate like a hand-held meter. It measures the light only at the center of the image. This means you can meter a person's face while completely ignoring the light of a bright window behind her for an accurate exposure.
It can also be used for manual exposure calculation. This lets you get specific readings for a person's face, the background, and the shadows, for example, and then to make a decision as to the final exposure.
Some cameras have several autoexposure choices, called program exposure modes. For example, there may be one setting for action shots that would always select a fast shutter speed. There also might be a program mode for landscapes or portraits. In this case, the camera automatically makes the settings based not only on the light but also other factors appropriate to the shot.