All of us have a need to capture screen shots (a/k/a Screen Dump, Screen Grab, Screen Cap[ture], PrntScrn, etc) at one point or another. Some might need to do this many times a day, others, maybe only once or twice a year.
As I'm a Maccer, I'll point you toward a nice summary of the OS X-native capabilities, along with a few suggestions for specialty utilities to use when the native grabs won't do what you want.
Please see the following 2 page article at the MacDevCenter on O'Reilly.com:
For the Windows users who read this: If you have a link to a similar article for your platform, please either email me or post a note in the Lounge and I'll add it to this thread.