TechEssence.info

OPML stands for Outline Processing Markup Language. It is an XML language used for marking up multi-level lists—that is, outlines.

Its chief use currently is as an interchange format for blogrolls. Bloglines, for example, imports and exports blogrolls in OPML format.

OPML is controlled by Dave Winer, who has also been involved in nasty and protracted format wars over RSS. An up-and-coming competitor to OPML may be XOXO, which uses XHTML tags instead of OPML's home-grown ones to delineate outline levels.