Outline document written in OPML (Outline Processor Markup Language); used to exchange lists of Web feeds (such as .RSS files) between Web feed aggregators; also used for other general outlining purposes.

Each OPML file contains the following XML elements:

1. OPML version - e.g. "1.0"
2. Head - contains metadata that describes the content
3. Body - contains the content of the outline
4. Outline - contains a line in the outline