Windows:
In Microsoft Windows a screenshot of the entire monitor, complete with taskbar, can be copied to the system clipboard by pressing the Print screen key. Alternatively, pressing Alt + Print screen will copy just the active window to the clipboard. One can then paste the clipboard into a program such as MS Paint or Paint.NET to save it as an image file (for posting online, for instance), or paste it directly into a document.
Mac OS X:
'⌘' Key is located on the left side of the space bar on your keyboard.
Take a picture of the whole screen
⌘-Shift-3
Take a picture of part of the screen
⌘-Shift-4, then drag to select the area you want in the picture.
To cancel, press Escape.
Take a picture of a window, a menu, the menu bar, or the Dock.
Press ⌘-Shift-4, then press the Space bar. Move the pointer over the area you want so that it's highlighted, then click.
To drag to select the area instead, press the Space bar again. To cancel, press Escape.
Screenshots are saved as files on the desktop. If you want to put the screenshot in the Clipboard, rather than create a file, hold down the Control key when you press the other keys. You can then paste the picture into a document.
You can also take pictures of the screen using the Grab application (in the Utilities folder).