Wikipedia features 3 type of tooltips: inline tooltips added by editors, page preview tooltips for wikilinks, and reference tooltips.

Editors can add inline tooltips to articles using the ((Tooltip)) template, like so: ((Tooltip|The text|The tool tip)) which makes it appear like this: The text.

The ((Abbr)) template can be used for abbreviations. Tooltips are also used by various templates. For instance, the ((obsolete source)) template, here shown with a tooltip reason: ^{[obsolete source]}.

In general, editors should use inline tooltips selectively such as when they're useful for table column headers. (See MOS:NOHOVER for information)

Editors should check that tooltip text renders as intended. For instance, if the tooltip text includes a " character, the tooltip is cut short.

When hovering over a link to another article, a short summary of the subject and an image (if available) is displayed as a tooltip.

When hovering over a reference, a tooltip appears that shows the full reference.

• Wikipedia:Tools/Navigation_popups
• mw:Extension:Tooltips
• mw:Extension:Tooltip