P.18 Tooltips (or "Balloon Help")

Tooltips are small X windows displaying a help string at the current mouse position, typically over text--including the mode line--which can be activated with the mouse or other keys. (This facility is sometimes known as balloon help.) Help text may be available for menu items too.

To use tooltips, enable Tooltip mode with the command M-x tooltip-mode. The customization group tooltip controls various aspects of how tooltips work. When Tooltip mode is disabled, the help text is displayed in the echo area instead.

As of Emacs 21.1, tooltips are not supported on MS-Windows. So help text always appears in the echo area.