Some camera’s have multiple metering modes for the built in light meter. My camera for example has Spot metering, Partial, Center weight average, and Average metering. These different metering modes help us compensate for different situations and help us get better exposure’s if we use them properly.
Metering Modes – Canon
Basically how the camera light meter works is it tries to expose for 18% gray. In other words if you point at white snow the camera’s light meter will adjust so it exposes the snow at a 18% gray value. Therefore the resulting image will be Under Exposed, but according to the camera it’s a correct exposure. We know better though 
So knowing this 18% gray info, can you predict what the camera’s going to do when you try and take a picture of a dark black train???
It will Over Expose it because it tries to make the black 18% gray. So knowing this we can simple adjust the shutter speed a full stop under what the camera tells us to use. That should yield a pretty accurate result
Evaluative: Metering is directly linked to, and concentrated on, the active Auto focus (AF) point. Light values measured at the active AF point are compared with light values measured from the metering segments surrounding the active point, and the camera’s metering system attempts to provide an accurate exposure based on that comparison.
Spot Metering: This metering mode gets exposure information only from the single exposure zone in the center of the frame (approximately 3% of the total picture area) Spot metering is great for for finding exactly what brightness a given area in the scene is.
I use this all the time when shooting HDR photography to find out what I need my shutter speeds to be in order to capture all the dynamic range in the scene. Basically I put the camera on spot meter and the Aperture I want. Then as I pan around the scene the shutter speed will automatically change as I move around, and it’s very accurate.
Partial Metering: This metering mode is similar to Spot Metering, but covers a slightly larger area, reading only the cross-shaped central five metering zones (approximately 10% of the total picture area) Great for Natural light Portraits!
Center-weighted Average Meteringh: This metering mode averages the exposure for the entire picture area, but with greater emphasis on the center metering zones.
For my purposes I usually use Evaluate metering as it does a good job with regular snapshots and it evaluates based on your focus point which works great most of the time. For HDR photography I use Evaluate mostly, but occasionally I will use spot metering to get accurate readings across the scene. For studio photography work I always use manual mode and evaluate metering. For natural light portrait shooting I use partial metering and/or center weight average metering because I want to make sure the subjects face is exposed properly. Evaluate mode tends to blend the background and subject a bit more than I like leading to an under exposed face a lot of the time in my experience
another awesome write up. thanks!