Our eyes are very complex and versatile lenses which are able to perceive various degrees of light and adjust to correctly expose accurately all of them. For example, if there is a direct and bright sunlight behind a friend our eyes can differentiate the face even though it is in a shadow.
However, a Camera can adjust exposure for an overall scene. Since at any given situation, there are varied forms of lights, the Photographer must instruct the Camera how to Meter (read the Light) a Scene. The photographer instructs whether to expose the Face or the Sunlight or something in between the two extremes. Metering assesses the amount of light in a scene and correspondingly uses the required information to adjust the Shutter Speed, Aperture and ISO in order to correctly expose the Photograph.
The Metering can adjust the variables of Shutter Speed, Aperture or ISO in order to rightly expose a Photograph. However, which variable the Camera will adjust will depend on the mode of Shooting. If you are on a Shutter Priority mode with Auto ISO, the Camera will adjust the Aperture and ISO. If the ISO is fixed to a particular value, the Camera will have only one variable i.e. Aperture to work with. Similarly, in an Aperture Priority mode, the Camera will adjust the Shutter Speed and ISO. Note, if you are shooting in Full Manual Mode, where you control the Shutter Speed, Aperture and ISO, the Metering will not work, since there is no variable left for adjustment. Still, through your Viewfinder or LCD screen you will have an option to adjust exposure (playing around with the Shutter Speed, Aperture and ISO) the based on the Metering mode you have set.
Before moving onto the various types of Metering Modes, two forms of Lights need to be understood. Incident Light is the light that falls on a subject, either directly or indirectly. Direct sources of incident illumination are sources that emit light, like the sun. Indirect sources of incident illumination redirect light onto the subject like a wall from which sunlight bounces back onto the subject. Reflected Light is the light that bounces off your subject and other elements in the scene. Whatever type of lighting you choose to create an image, it is imperative to meter the scene to obtain the correct exposure. Most DSLRs are built to Meter the Reflected Light.
Matrix / Evaluative Metering
Matrix Metering (Nikon) and Evaluative Metering (Canon) is often the default setting on most DSLRs, this method works best on scenes which are fairly evenly lit. The camera divides the frame in to zones and individually measures their exposure, adding bias to any zones which have active focus points. It then takes all this information and makes a calculation on the best overall exposure for the entire image. The exact algorithm for calculation is a secret and varies from one Manufacturer to the other. This is the mode to use when you’re not too sure which mode the scene demands.
Like Matrix / Evaluative metering, Centre-weighted metering measures light across the whole frame but adds approximately 70% bias towards the centre. This is a great option when the subject is in the center of the photograph and need to be exposed correctly, so that the subject is not affected by the exposure of the background. Centre-weighted metering tells the camera to concentrate on the center, but also considers the background exposure. Centre-weighted metering is generally the default metering in Point & Shoot Cameras since most users center the Subject and generally do not consider composition techniques like the Rule of Thirds.
Spot / Partial Metering
This is often the second most popular setting behind matrix / evaluative metering. Spot metering takes a measurement at the active focus point. Approximately 1-3% of the frame is metered, so this method is very useful if you want to expose for one specific point e.g. in shooting Portraits. Here you would spot meter on your subject’s face or eyes to ensure an accurate exposure, thus ignoring the background. This mode is useful when you have a very specific area of the photograph that you wish the exposure to be calculated upon.
A Light Meter is a Camera independent device used to measure the amount of light and it calculates the Shutter Speed, Aperture & ISO. The settings are replicated in Camera to get the perfect Exposure. They are very useful in Commercial Photography. However, if you feel a little adventurous and own an Android Phone you can download a few Light Meter Apps (Free and Paid) at Google Play Store to convert your Phone to a Light Meter. The exposure values suggested by them work fairly well.