Tooltips are boxes that pop up when a user hovers (or clicks) on a link. They can be used to add context to an external link – for example to advise that the link leads off-site or will play a video or sound.
They are also useful as a substitute for footnotes in longer articles. Use them to cite a source in a less obtrusive way, or add a comment to a word or phrase. The following demo shows a lightweight tooltip plugin that is easy to use and reader-friendly. I especially like them in article where a lot of acronyms or foreign words are found – it is easy to hover over the link to display the expanded meaning of the word.