The WordPress eXtended Rss (WXR) Export/Import, XML Document Format Decoded and Explained
devtidbits.com/2011/03/16/the-wordpress-extended-rss-wxr-exportimport-xml-document-format-decoded-and-explained
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
The WXR format is based on the Really Simple Syndication or Rss specification which is a very popular dialect of XML. It has been designed as a syndication format for websites who wish to share and serialise some of their data.