Taking screenshots on your Mac is easy... just hold down Command+Shift+4 and then drag a square around what you want to screenshot. An image name "screenshot" will then be saved to your desktop. There are other keystrokes you can use depending on whether you want the whole screen, a specific window or just want the image copied to the clipboard. Learn more here: http://support.apple.com/en-us/HT201361
Sometimes, however, you want to take that screenshot and add annotations to it. That's where a free application called Snip can come in handy.
With Snip, you can draw squares, circles, arrows and lines or type text directly onto your screenshot. Then, when your image is ready, click the download icon (the last icon on the toolbar) to save the image to your desktop.