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Exposure compensation makes your captured image brighter or darker overall. You would want to use +1.5 to +2 if your subject was strongly backlit, like a person inside with mostly a bright window behind them. You would want to use -1.5 or so if your image had mostly black background, like flowers on a black tablecloth. Your camera's metering will try to make the scene it sees a flat neutral grey, on average, and you can normally use a "gray card" to set your metering in tricky situations. Some cameras have matrix type metering which uses a database of exposures taking multiple light readings to compensate for tricky lighting conditions. Play around with exposure compensation and take a few pix of the same thing with different settings, then look at them and a histogram in your photo editing program... you will find that underexposed images become noisy when you make them brighter with software, and overexposed highlights become pasty and odd looking when you make the image dark enough to look good with editing software.
btw, you can use a program like photomatix or photoshop to combine images shot with different exposures to create an image which appears to show a larger dynamic range. This is called HDR. You normally want a tripod to do this and you take an image each at -2, 0, and +2 of a non-moving subject. There are probably some free HDR solutions, but normally most people like photomatix for this, photoshop cs2 and later have HDR tools built in but they are less powerful... google is your friend.
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