With the HTML type, you can display just about any kind of HTML code. It will show as a popup box near the button.

Custom Button

When all other types fail, Custom Button is your last resort.

Parameters

Script Location: URI of a script file
JavaScript: JavaScript statement (added to the page HEAD)
onClick: Function to attach to button.

Note that unless Joomla! decides to use something else, the MooTools library will always be loaded. Thus, you can use it without any problem.