How to Take a Snapshot of a Mac's Screen. Here's an easy way to record a portion of your Mac's screen, such as an error dialog box or a thumbnail graphic on the Web.
Press Command + Shift + 3. When you hear a click, your Mac has taken a picture. This picture can be found on your hard drive with the file name "Picture 1," or a subsequent number if you've snapped more than one picture.
Double-click the picture icon to launch SimpleText. This will show you what you've snapped. You can select a portion of this picture with your pointer in order to copy it into a document, or you can open the file in a graphics application to perform more sophisticated editing.