Take a screenshot on a Mac: Keyboard shortcuts
Several keyboard shortcuts enable you to take a screenshot on a Mac.
Holding down Cmd, Shift, 3 will grab the entire screen, including your desktop.
Holding down Cmd, Shift, 4 will display crosshairs onscreen, allowing you to drag over the area you want to capture. In Leopard or later you can then press Space bar to move the selected region with the mouse; Shift to resize one edge of the selected area; or Alt to resize the entire area. Release the mouse to capture the screenshot.
Holding down Cmd, Shift, 4, then hitting the Space bar once the crosshairs have appeared onscreen (but before you select the capture area) will enable you to click on an open window and snag only that portion of the screen.
Add Ctrl to any of these keyboard shortcuts to save the resulting screenshot to the clipboard rather than the desktop.
Take a screenshot on a Mac: Grab utility
OS X has a built-in screenshot utility called Grab. You'll find it in the Utilities folder within Applications.
Launch Grab, then choose the Capture menu. Here you can choose to capture a selected area, a window, the entire screen or a timed screen option. The various keyboard shortcuts for each option are displayed beside each capture type: Shift, Cmd, A for Selection; Shift, Cmd, W for Window; Cmd, Z for Screen; Shift, Cmd, Z for Timed Screen.
Take a screenshot on a Mac: Terminal
A slightly geeky and perhaps unnecessary way to take screenshots on a Mac is to use the Terminal. Open the application, then type in one of the following statements and press enter.
To take a screenshot of the entire screen: screencapture -S ~/Desktop/screen.jpg
To take a screenshot of a selected window and save it to the desktop: screencapture -iW ~/Desktop/screen.jpg
To take a screenshot of a selected area and save it to the desktop: screencapture -ic
Take a screenshot on a Mac: App
Plenty of third-party apps let you take and edit screenshots on a Mac. Just open the Mac App Store, search for screenshot, then take your pic from the free and paid-for options available.