This is probably the most complex metering mode, offering the best exposure in most circumstances. Essentially, the scene is split up into a matrix of metering zones which are evaluated individually. The overall exposure is based on an algorithm specific to that camera, the details of which are closely guarded by the manufacturer. Often they are based on comparing the measurements to the exposure of typical scenes.
Probably the most common metering method implemented in nearly every digital camera and the default for those digital cameras which don’t offer metering mode selection. This method averages the exposure of the entire frame but gives extra weight to the center and is ideal for portraits.
Spot metering allows you to meter the subject in the center of the frame (or on some cameras at the selected AF point). Only a small area of the whole frame is metered and the exposure of the rest of the frame is ignored. This type of metering is useful for brightly backlit, macro, and moon shots.