OPML (Outline Processor Markup Language) is an XML format for outlines. Originally developed by Radio UserLand as a native file format for an outliner application it has since been adopted for other uses, the most common being to exchange list of RSS feeds between RSS aggregators.
The OPML specification defines an outline as a hierarchical, ordered list of arbritrary elements. The specification is fairly open which makes it suitable for many types of list data
full details can be found at
and complete definition is listed here