With metering modes its how much of the lense/scene the camera uses to conduct light metering.
Evaluative: this is where the camera divides the whole scene into sections and calculates an average exposure based on the readings in the from each section. I generally use this when shooting landscapes.
Center weighted: I'm not too sure how much of the scene the camera uses to come up with a light meter reading but basically its a large area centred around the centre of your scene.
Partial: Again centered on the middle of the scene it covers a much smaller area about 10% of the scene.
Spot: Again centered on the middle of the scene but it covers a area of about 4% of the scene.
Hope you can follow that ?????
Generally when using spot or partial metering I always ensure the center autofocus point is over the top the subject. If the image is going to require the subject to be off centered then meter over the subject using the center autofocus point, then use exposure lock to retain the exposure settings then reframe the image.
If you want to edit any of my images to demonstrate points/techniques to either myself or others please feel free to do so.
Still learning with Canon 40D, 28-135mm, 100-400mm L and Elements 6.
Any Advice greatfully received.