Courtesy of MacWorld:
"The basics of taking a Mac screenshot are very simple:
1. Hold Cmd + Shift, then press 4
2. Drag crosshairs across area of screen you want to screenshot
3. Screenshot appears on your desktop as a .png file, labelled as 'Screen Shot' [year]-[month]-[day] at [time]'
Holding down both Cmd (also known as the 'Apple key') and Shift, then pressing 3, will instantly grab a screen capture or screenshot of the entire Mac desktop, which will appear on the desktop as a .png image whose filename will begin with 'Screen shot' and the date and time. If you're using two screens with your Mac, two images will appear - one for each display.