Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Command-shift-3: Take picture of entire screen
Command-shift-4, then select area: Take picture of specific area
Command-shift-4, then space, then click a window: Take picture of a window

Hold down control during any of these to put the screenshot on the clipboard instead of as a file on the desktop.
 
Duff-Man says....there is a program in your applications/utilities folder called "Grab" - this will also work you. Also, try using the built in Mac Help - all this stuff is in there and you don't have to wait for responses!....oh yeah!
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.