The metering system in a digital camera measures the amount of light in the scene and calculates the best-fit exposure value based on the metering mode. There are 3 types of metering mode:

1. Overall Metering (Multi Segment/Zone Metering)
2. Spot/Partial Metering 
3. Center Weighted Metering

Some links about metering:

• digital photography school
• dpreview
• ephotozine
• weather scapes