The Mac always had a screenshot feature. Pressing 'cmd-shift-3' simultaneously would create a screenshot even in System 1. As of OSX, 'cmd-shift-4' will also create a screenshot, but with the added benefit that you can crop the area before OSX saves it. In Panther (10.3) and earlier your Mac saves screenshots in PDF format, in Snow Leopard (10.6) in PNG. In all versions, screenshots are saved to your 'Desktop' folder ((Home)/Desktop).
If you had Unix knowhow, nerves of steel and the grim determination to do it mano-a-mano (whatever that is), you always could change file format and save location by entering a string of arcane commands (see ''Real' men do it via terminal' how do tough it out old-skool-like). Savescreenie is a frontend that does that for you - without worrying if 'killall SystemUIServer' really will only restart the windows server, or if it instead would go about murdering some other processes it doesn't like.
Mac OS X 10.5 Intel
Mac OS X 10.5 PPC