command + shift + 4 will turn your cursor into cross-hairs and you can take a small segment of the screen.
command + shift + 4 + space bar will let you copy a single window.
all of these will automatically save a pdf file of the screen-shot to your desktop. adding control to the mix of any of them sends it to your clipboard for use later. [ Moop's advice column | Ask Moop A Question ]
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