Alternatively referred to as a tooltip, a balloon is a brief message that indicates what would occur if they were to click on that icon/button. Programs that support this feature displays the balloon when moving over an object and not clicking or moving the mouse. The picture shows an example of a balloon when hovering over the Start button in Microsoft Windows that shows "Click here to begin".
Balloons are also found in Internet browsers as alt text or title text. For example, when hovering your mouse over the picture to the right, you should see "Microsoft Windows balloon".