Shift+Apple+3 will take a screenshot of the entire screen and save it to your Desktop.
Shift+Apple+4 will change the cursor to a cross-hair for capturing a selection of the screen and save the resulting file to your Desktop.
Shift+Control+Apple+3 or 4 will do the same (full screen or selection) but will save the image to your clipboard to paste into a graphics application.
Shift+Apple+4 then hit the space bar will change the cursor to a camera, which can be used to take a shot of an entire window at once.
'cybergoober' suggests valid keyboard equivalent snapshot taking actions; however, the format of the snapshots depends on which version of MacOS X you are currently running.
'Panther' (10.3.x) saves the snapshots as '.pdf' files; whereas, 'Tiger' (10.4.x) saves them as '.png'.
'Snapz Pro X' does the same as, and more than, Apples provided keyboard equivalent snapshot taking actions. Some of its additional features include: setting the file format of the snapshots, where to save the snapshots (sadly - fixed and limited, many would like to see an 'Other...' option available), or even print them once taken. For an additional cost, you can even create '.mov' format movies.
You can also use Cmd-Ctrl-Shift-3 or Cmd-Ctrl-Shift-4. Those just put the screenshot into the clipboard, from where you can paste it into any graphics processor. (And save the file later in the format you want...) I usually use the latter solution, since I rarely need a shot of the whole screen.