Exposure

Basic Daylight Exposure

http://www.dummies.com/how-to/content/digital-photography-exposure-for-dummies-cheat-she.html

Exposure Tips

http://www.slrphotographyguide.com/camera/settings/exposure-tips.shtml

http://everydayaperture.com/sunny-16-and-beyond/

Middle Gray

Middle gray is the universal measurement standard in photographic cameras. To calibrate light meters, whether in a camera or hand held, the 18% gray card was conceived. It is assumed that the measurement taken by a meter gives the exposure for a shot so that some of the light reflected by the object measured is equivalent to middle gray.^[4] However, many note that modern cameras generally treat 12-13% gray as "middle gray".

18% gray card - rgb(124,124,124),  #7C7C7C

https://en.wikipedia.org/wiki/Middle_gray

Metering

Evaluative is the default setting.
The camera sets the exposure automatically to suit the scene, taking into consideration both dark and light areas. It's considered good for evenly backlit subjects like portraiture and landscapes. It's also the way to go when you're not sure which metering mode to use. This is the reason why it's the default setting for fully automatic camera settings.

Center weight setting
Center-weighted metering assigns the greatest weight for exposure from the middle area of the frame. Therefore, it's good for times when your main subject is in the middle of the frame and you want to take a quick exposure.

Center weighted metering would be effective when you have a bright background or backlit subject. For example, if you were taking a photograph of a persons face on a sunny day at the beach. You wouldn't want the strong background light, or the white sand, to effect the exposure on their face. As long as the persons face was correctly exposed, that's all that matters. It's times like this, you would choose center weighted metering.

Spot/Partial setting
Partial (Canon) metering should be used when you want to take an exposure reading on a specific area. It takes the reading from a very small area in the middle of your composition.  Use this for macro shots.
http://www.slrphotographyguide.com/spot-metering.shtml
http://www.slrphotographyguide.com/partial-metering.shtml

http://www.slrphotographyguide.com/camera/settings/metering-mode.shtml