Generally with modern cameras several exposure metering modes are available to select from. The metering is initiated by the half press of the shutter realease button.
In most cameras the following modes may be available.
Color Matrix Metering
In this mode metering can be performed by a sensor with up to 1,000 pixels and this sets the exposure depending on a variety of criteria. The uses information from the lens in adjusting exposure and takes into account the brightness and colors in the areas of the scene.
Center Weighted Metering
In this mode the camera meters the entire image frame but it assigns a weighting to the central area of the image. Generaly the weighted area in the image is indicated in the central area of the viewfinder frame.
The camera meters a tiny cincle in the center of the viewfinder frame.
The exposure modes determine how the camera adjusts the shutter speed and the aperture.
In this mode the camera automatically adjusts the shutter speed and the aperture for optimal exposure in most situations. But this moode will not take into account other issues such as depth of field and any subject movement. For the photographer to be in charge it is desired that other modes be chosen. Some cameras will also enable the photographer to make small adjustments to the program
Shutter Priority Auto
This mode the photographer selects the shutter speed and the camera will automatically select the correct aperture. This mode is useful to stop fast action or to create blurred effects using a slow shutter speed.
Aperture Priority Auto
This mode allows the photographer to select the aperture and the camera will select the correct shutter speed. This mode is very useful to control the depth of field in the image.
Manual mode is used to give the photographer full control over the complete exposure, shutter speed and aperture. This mode is very useful for night time shooting for subjects such as fireworks, lightning and night scenes generally.
For certain images it may be necessary to adjust the camers exposure for creative effects, such as strong back lighting situations or to correctly expose silhouettes.
Exposure bracketing is usefull when you may be unsure of the exposure or camera settings. The camera can then take several exposures as selected around the settings shown by the camera.
White Balance Bracketing
White balance bracketing can be used if you are unsure of the white balance settings and several images can be taken with different white balance settings.
It should be noted that with Digital cameras the adjustment of ISO is possible as an adjustment in parallel with shutter speed and aperture.
Auto ISO can be used very effectively to interact with the aperture and shutter speed settings, but try to keep the ISO as low as possible to avoid noisy images.