Camera metering modes as explained by Leading manufacturers

Center weighted

Nikon's center weighted

Camera meters entire frame but assign greatest weight to area in center of frame 8mm (0.31 inches) in diameter, shown by corresponding 8mm refrence circle in viewfinder. Classic meter for portraits. recommended when using filters with an exposure factor ( filter factor) over 1x. (sourced from D200 manual)

Canon's Partial metering

Effective when the background is much brighter than the subject due to backlighting, etc. PArtial metering covers about 8% of the viewfinder area at the center. (source EOS 5D manual)

Multi pattern metering

Nikon's 3D matrix metering

1005-pixel RGB sensor sets exposure based on variety of information from all areas based on variety of information from all areas of the frame. With type G or D lenses, camera uses 3-d colour matrix metering II for natural results even when frame is dominated by bright (white or yellow) or dark (black or dark green) colors. With other CPU lenses, 3D range information is not included, instead, camera uses color matrix metering II. Color matrix metering is available when focal length and maximum aperture of  non CPU lenses are specified using not CPU lens data item in shooting menu. Then center weighted metering is used if focal length or aperture is not specified. matrix metering will not produce desired results with auto exposure lock or exposure compensation, but is recommended in most other situations.(source-D200 manual)

Canon's Evaluative

This the camera's standard metering mode suited for most subjects even under backlit conditions. After detecting the main subjects position, brightness, background, front and backlighting, etc, the camera sets the proper exposure.(source EOS 5D manual)

Canon's Center weighted average metering
The metering is weighted at the center and then average fro entire scene.(source EOS 5D manual)