How to take a screenshot and write on it on a Mac

There are two simple keyboard combinations for taking a screenshot on a Mac:

• Command-Shift-3 (pressed all at the same time) takes a picture of the entire window. On older Macs, the Command Key was not labeled with "Command" but had two icons: a cloverleaf and an "Open Apple".
  
  

• Command-Shift-4 (pressed all at the same time) allows you to choose which part of the screen gets captured using your mouse

To annotate a screenshot, drag the .png screenshot file from your desktop onto the Preview icon in your dock to open it up. Click on the Annotate button on the top toolbar and look for the drawing icons on the bottom left corner of your screen.  There you can choose to make a circle, rectangle, text box, arrow, line and change colors of these items as you mark up your screenshot.  A picture is worth a thousand words, so why not annotate a screenshot rather than write a thousand words?

Update 9/11/15 To take screenshots on platforms other than Mac, go to http://www.take-a-screenshot.org/.