This article (link) gives some great tips on taking screenshots in OSX. By default, all screenshots are saved as .png files to the desktop.

Command-Shift-3 Screenshot of whole screen
Command-Shift-4 Changes cursor into crosshair, can then select an area, click to take snapshot
Command-Shift-4 then Space Changes cursor into camera, hover over object to take shot of only that object with mouse-click
Command-Shift-4 then Space Same as before, but Option-click takes a shot without the shadow

I was originally searching for how to take a screenshot without the 'hover shadow' when I came across the link above.