On Mac OS X, a user can take a screenshot of an entire screen by pressing ⌘ Cmd+⇧ Shift+3,

Or of a chosen area of the screen by ⌘ Cmd+⇧ Shift+4.

This screenshot is saved to the user's desktop, with one PNG file per attached monitor.

If the user holds down Ctrl while doing either then the screenshot will be copied to the clipboard instead.