Ok, time to get into the good stuff.
Metering, is the technique you use to achieve perfect exposure.
The camera reads light intensity as a value between 0-255. 0 being complete darkness, and 255 being complete light/whiteness. By metering, you tell your camera what value is completely in the middle of these two values.
Look at the picture above. If I had "spot metered" the picture as the shadows in the bottom, then the black would be considered the "middle intensity" and the camera would expose so the shadows appeared as the middle intensity (which is a color called 18% grey). The problem comes from that a lot of data: the green grass, sidewalk, sunlight, would all be pushed up to 255 light intensity, and would appear as pure white in the data file. There is no way to recover that data, as it is all read as white. Thus, we need to understand metering.
Theres different ways to meter, superior knowledge, a light meter, a gray card, or using your cameras metering system. I use my camera's built in metering system because it does enough for the type of shots I want. First, lets go over the types of metering:
Evaluative: The camera examines the whole view, and then averages it so that it is neither overexposed and underexposed. This works well in most situations, unless you want to start shooting into the sun, or shooting where theres areas of darkness and brightness in the same picture.
Spot metering: The camera meters for the focus spot you have selected as your focus point (usually center focus)
Partial metering: The camera meters off a larger area (10-15% of the frame) around a selected point.
Center weighted metering: The camera meters 60-80% off the center of the frame, and then feathers out the weight of it towards the edges of the frame.
Average, partial, spot, and center weighted metering on a Canon 5D Mark 2
To be honest, I don't have experience with partial and center weighted. I stick to spot metering.
A few tips my mentor Dan Carter taught me:
What we are usually trying to meter in sunlight is 18% grey. If you dont have a grey card on you, green grass is usually 18% grey. So before you take the picture, point the camera at some grass in the same sunlight as your subject, click the metering button which locks in your exposure, and go back to the subject and take the picture.
Spot metering lets you capture images most other cant. Say you want to shoot the subject with the sun behind her. Usually a big problem with point and shoots, since they average meter and youll end up with a completely black subject, with a light sky. Instead, use spot metering to expose off the person's skin, and then take the picture with the sun outside of the frame. There will be some lens flare and washout of the subject, but this can be fixed in post processing.
For the photo above, I spot metered for the grass halfway between me and the tree in Aperture priority mode. Then I just snapped the picture, added some warmth in post processing, and thats it!