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[ ] is center weighted average, good for portraits and such where you want the subject (center) to be the best exposed, it weighs more exposure decision on the center but then average the rest of the scene as well.
[+] is spot metering, good for macros, wildlife, where you want the exposure to be for the subject only basically and the rest of the scene may or may not be exposed properly
the other mode is evaluative, which is the one you would use for landscapes and such that you want the entire scene exposed properly.
Not the best or most technical explanations, but should give you a basic idea. The following websites will help you even more: