Tooltips can be attached to any element. When you hover the element with your mouse, the title attribute is displayed in a little box next to the element, just like a native tooltip.
But as it's not a native tooltip, it can be styled. Any themes built with ThemeRoller will also style tooltip's accordingly.
Tooltip's are also useful for form elements, to show some additional information in the context of each field.
Click the field to see the tooltip; when you tab out of the field, it gets hidden.
This demo shows how to customize animations. The tooltip is shown, after a delay of 250ms, using a slide down animation, and hidden, after another delay, without an animation.