Have you ever taken a picture and wondered why in the world your subject was underexposed when your metering said it was perfectly exposed? The reason could be that you are not using the proper metering mode.
Why re-invent the wheel? Check out this explanation of the 3 different metering modes.
There are different types of metering and they are called different things on a Canon verses a Nikon. Here is an example of each of the metering modes. All of these are SOOC for the purpose of you seeing the difference. I metered off of her skin and my meter stated that all of these were properly exposed.
Personally, I use spot metering about 99% of the time. I meter off of the skin by placing my focal point on their face.
Metering mode is a personal choice depending on the situation so feel free to play around with your modes to figure out what you like best.