Jing is a screen capture application that allows you to select and capture parts of your screen and save that area as a static picture or even a movie.
Then you can add arrows, text and box highlights to draw further attention to the area you want to highlight.
The thing that makes Jing unique is that it has a "share" button, which allows you to share the image or video with a friend by connecting to the free screencast.com image hosting solution. Click share, it uploads your media behind the scenes, and places the link to the screen in your clipboard. You can then email or paste that URL into your chat application for your friends to see.