Ruby text is small-font annotation that usually appears above or below the main body text (or to one side in vertically-oriented writing systems). The name comes from a small font that was used in typography to create the small annotation text. Ruby text is more commonly employed in pictographic languages, where the ruby text supplies a pronounciation guide to the main text pictographic symbols. But ruby text can be used with Latin alphabet languages, too.

The ruby element is a master container for all content to be affected by ruby markup, including the main text. The main text is known as the ruby base, while the annotation is called ruby text. Each of these types has its own tag (rb and rt, respectively), and any such tags must be encased within a ruby element. IE implemented the basics of ruby markup starting with Version 5 (Windows and Mac).

The W3C ruby markup specification was developed independently of the HTML recommendation, and was added to XHTML 1.1 as one of the first modules to take advantage of the extensible nature of XHTML.

The language being used for the element's attribute values and content. A browser can use this information to assist in proper rendering of content with respect to details such as treatment of ligatures (when supported by a particular font or required by a written language), quotation marks, and hyphenation. Other applications and search engines might use this information to aid the selection of spell-checking dictionaries and the creation of indices.